user-level: 0.970
mistranslation: 0.962
risc-v: 0.960
kernel: 0.941
debug: 0.939
arm: 0.934
device: 0.931
graphic: 0.931
permissions: 0.927
assembly: 0.926
PID: 0.926
architecture: 0.925
register: 0.924
semantic: 0.923
hypervisor: 0.920
performance: 0.920
virtual: 0.917
KVM: 0.914
VMM: 0.909
socket: 0.907
vnc: 0.905
network: 0.904
boot: 0.895
peripherals: 0.895
operating system: 0.894
TCG: 0.889
files: 0.884
alpha: 0.873
ppc: 0.842
i386: 0.822
x86: 0.805
--------------------
virtual: 0.897
hypervisor: 0.757
KVM: 0.717
debug: 0.694
socket: 0.541
kernel: 0.452
TCG: 0.366
x86: 0.260
network: 0.208
register: 0.159
operating system: 0.097
device: 0.063
PID: 0.049
files: 0.034
VMM: 0.031
risc-v: 0.023
assembly: 0.017
ppc: 0.015
alpha: 0.011
peripherals: 0.010
user-level: 0.009
i386: 0.008
semantic: 0.008
performance: 0.007
architecture: 0.007
graphic: 0.004
permissions: 0.004
arm: 0.003
vnc: 0.003
boot: 0.003
mistranslation: 0.001

[BUG, RFC] cpr-transfer: qxl guest driver crashes after migration

Hi all,

We've been experimenting with cpr-transfer migration mode recently and
have discovered the following issue with the guest QXL driver:

Run migration source:
>
EMULATOR=/path/to/emulator
>
ROOTFS=/path/to/image
>
QMPSOCK=/var/run/alma8qmp-src.sock
>
>
$EMULATOR -enable-kvm \
>
-machine q35 \
>
-cpu host -smp 2 -m 2G \
>
-object
>
memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/ram0,share=on\
>
-machine memory-backend=ram0 \
>
-machine aux-ram-share=on \
>
-drive file=$ROOTFS,media=disk,if=virtio \
>
-qmp unix:$QMPSOCK,server=on,wait=off \
>
-nographic \
>
-device qxl-vga
Run migration target:
>
EMULATOR=/path/to/emulator
>
ROOTFS=/path/to/image
>
QMPSOCK=/var/run/alma8qmp-dst.sock
>
>
>
>
$EMULATOR -enable-kvm \
>
-machine q35 \
>
-cpu host -smp 2 -m 2G \
>
-object
>
memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/ram0,share=on\
>
-machine memory-backend=ram0 \
>
-machine aux-ram-share=on \
>
-drive file=$ROOTFS,media=disk,if=virtio \
>
-qmp unix:$QMPSOCK,server=on,wait=off \
>
-nographic \
>
-device qxl-vga \
>
-incoming tcp:0:44444 \
>
-incoming '{"channel-type": "cpr", "addr": { "transport": "socket",
>
"type": "unix", "path": "/var/run/alma8cpr-dst.sock"}}'
Launch the migration:
>
QMPSHELL=/root/src/qemu/master/scripts/qmp/qmp-shell
>
QMPSOCK=/var/run/alma8qmp-src.sock
>
>
$QMPSHELL -p $QMPSOCK <<EOF
>
migrate-set-parameters mode=cpr-transfer
>
migrate
>
channels=[{"channel-type":"main","addr":{"transport":"socket","type":"inet","host":"0","port":"44444"}},{"channel-type":"cpr","addr":{"transport":"socket","type":"unix","path":"/var/run/alma8cpr-dst.sock"}}]
>
EOF
Then, after a while, QXL guest driver on target crashes spewing the
following messages:
>
[   73.962002] [TTM] Buffer eviction failed
>
[   73.962072] qxl 0000:00:02.0: object_init failed for (3149824, 0x00000001)
>
[   73.962081] [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to allocate
>
VRAM BO
That seems to be a known kernel QXL driver bug:
https://lore.kernel.org/all/20220907094423.93581-1-min_halo@163.com/T/
https://lore.kernel.org/lkml/ZTgydqRlK6WX_b29@eldamar.lan/
(the latter discussion contains that reproduce script which speeds up
the crash in the guest):
>
#!/bin/bash
>
>
chvt 3
>
>
for j in $(seq 80); do
>
echo "$(date) starting round $j"
>
if [ "$(journalctl --boot | grep "failed to allocate VRAM BO")" != ""
>
]; then
>
echo "bug was reproduced after $j tries"
>
exit 1
>
fi
>
for i in $(seq 100); do
>
dmesg > /dev/tty3
>
done
>
done
>
>
echo "bug could not be reproduced"
>
exit 0
The bug itself seems to remain unfixed, as I was able to reproduce that
with Fedora 41 guest, as well as AlmaLinux 8 guest. However our
cpr-transfer code also seems to be buggy as it triggers the crash -
without the cpr-transfer migration the above reproduce doesn't lead to
crash on the source VM.

I suspect that, as cpr-transfer doesn't migrate the guest memory, but
rather passes it through the memory backend object, our code might
somehow corrupt the VRAM.  However, I wasn't able to trace the
corruption so far.

Could somebody help the investigation and take a look into this?  Any
suggestions would be appreciated.  Thanks!

Andrey

On 2/28/2025 12:39 PM, Andrey Drobyshev wrote:
Hi all,

We've been experimenting with cpr-transfer migration mode recently and
have discovered the following issue with the guest QXL driver:

Run migration source:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-src.sock

$EMULATOR -enable-kvm \
     -machine q35 \
     -cpu host -smp 2 -m 2G \
     -object 
memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/ram0,share=on\
     -machine memory-backend=ram0 \
     -machine aux-ram-share=on \
     -drive file=$ROOTFS,media=disk,if=virtio \
     -qmp unix:$QMPSOCK,server=on,wait=off \
     -nographic \
     -device qxl-vga
Run migration target:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-dst.sock
$EMULATOR -enable-kvm \
-machine q35 \
     -cpu host -smp 2 -m 2G \
     -object 
memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/ram0,share=on\
     -machine memory-backend=ram0 \
     -machine aux-ram-share=on \
     -drive file=$ROOTFS,media=disk,if=virtio \
     -qmp unix:$QMPSOCK,server=on,wait=off \
     -nographic \
     -device qxl-vga \
     -incoming tcp:0:44444 \
     -incoming '{"channel-type": "cpr", "addr": { "transport": "socket", "type": "unix", 
"path": "/var/run/alma8cpr-dst.sock"}}'
Launch the migration:
QMPSHELL=/root/src/qemu/master/scripts/qmp/qmp-shell
QMPSOCK=/var/run/alma8qmp-src.sock

$QMPSHELL -p $QMPSOCK <<EOF
     migrate-set-parameters mode=cpr-transfer
     migrate 
channels=[{"channel-type":"main","addr":{"transport":"socket","type":"inet","host":"0","port":"44444"}},{"channel-type":"cpr","addr":{"transport":"socket","type":"unix","path":"/var/run/alma8cpr-dst.sock"}}]
EOF
Then, after a while, QXL guest driver on target crashes spewing the
following messages:
[   73.962002] [TTM] Buffer eviction failed
[   73.962072] qxl 0000:00:02.0: object_init failed for (3149824, 0x00000001)
[   73.962081] [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to allocate 
VRAM BO
That seems to be a known kernel QXL driver bug:
https://lore.kernel.org/all/20220907094423.93581-1-min_halo@163.com/T/
https://lore.kernel.org/lkml/ZTgydqRlK6WX_b29@eldamar.lan/
(the latter discussion contains that reproduce script which speeds up
the crash in the guest):
#!/bin/bash

chvt 3

for j in $(seq 80); do
         echo "$(date) starting round $j"
         if [ "$(journalctl --boot | grep "failed to allocate VRAM BO")" != "" 
]; then
                 echo "bug was reproduced after $j tries"
                 exit 1
         fi
         for i in $(seq 100); do
                 dmesg > /dev/tty3
         done
done

echo "bug could not be reproduced"
exit 0
The bug itself seems to remain unfixed, as I was able to reproduce that
with Fedora 41 guest, as well as AlmaLinux 8 guest. However our
cpr-transfer code also seems to be buggy as it triggers the crash -
without the cpr-transfer migration the above reproduce doesn't lead to
crash on the source VM.

I suspect that, as cpr-transfer doesn't migrate the guest memory, but
rather passes it through the memory backend object, our code might
somehow corrupt the VRAM.  However, I wasn't able to trace the
corruption so far.

Could somebody help the investigation and take a look into this?  Any
suggestions would be appreciated.  Thanks!
Possibly some memory region created by qxl is not being preserved.
Try adding these traces to see what is preserved:

-trace enable='*cpr*'
-trace enable='*ram_alloc*'

- Steve

On 2/28/2025 1:13 PM, Steven Sistare wrote:
On 2/28/2025 12:39 PM, Andrey Drobyshev wrote:
Hi all,

We've been experimenting with cpr-transfer migration mode recently and
have discovered the following issue with the guest QXL driver:

Run migration source:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-src.sock

$EMULATOR -enable-kvm \
     -machine q35 \
     -cpu host -smp 2 -m 2G \
     -object 
memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/ram0,share=on\
     -machine memory-backend=ram0 \
     -machine aux-ram-share=on \
     -drive file=$ROOTFS,media=disk,if=virtio \
     -qmp unix:$QMPSOCK,server=on,wait=off \
     -nographic \
     -device qxl-vga
Run migration target:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-dst.sock
$EMULATOR -enable-kvm \
     -machine q35 \
     -cpu host -smp 2 -m 2G \
     -object 
memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/ram0,share=on\
     -machine memory-backend=ram0 \
     -machine aux-ram-share=on \
     -drive file=$ROOTFS,media=disk,if=virtio \
     -qmp unix:$QMPSOCK,server=on,wait=off \
     -nographic \
     -device qxl-vga \
     -incoming tcp:0:44444 \
     -incoming '{"channel-type": "cpr", "addr": { "transport": "socket", "type": "unix", 
"path": "/var/run/alma8cpr-dst.sock"}}'
Launch the migration:
QMPSHELL=/root/src/qemu/master/scripts/qmp/qmp-shell
QMPSOCK=/var/run/alma8qmp-src.sock

$QMPSHELL -p $QMPSOCK <<EOF
     migrate-set-parameters mode=cpr-transfer
     migrate 
channels=[{"channel-type":"main","addr":{"transport":"socket","type":"inet","host":"0","port":"44444"}},{"channel-type":"cpr","addr":{"transport":"socket","type":"unix","path":"/var/run/alma8cpr-dst.sock"}}]
EOF
Then, after a while, QXL guest driver on target crashes spewing the
following messages:
[   73.962002] [TTM] Buffer eviction failed
[   73.962072] qxl 0000:00:02.0: object_init failed for (3149824, 0x00000001)
[   73.962081] [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to allocate 
VRAM BO
That seems to be a known kernel QXL driver bug:
https://lore.kernel.org/all/20220907094423.93581-1-min_halo@163.com/T/
https://lore.kernel.org/lkml/ZTgydqRlK6WX_b29@eldamar.lan/
(the latter discussion contains that reproduce script which speeds up
the crash in the guest):
#!/bin/bash

chvt 3

for j in $(seq 80); do
         echo "$(date) starting round $j"
         if [ "$(journalctl --boot | grep "failed to allocate VRAM BO")" != "" 
]; then
                 echo "bug was reproduced after $j tries"
                 exit 1
         fi
         for i in $(seq 100); do
                 dmesg > /dev/tty3
         done
done

echo "bug could not be reproduced"
exit 0
The bug itself seems to remain unfixed, as I was able to reproduce that
with Fedora 41 guest, as well as AlmaLinux 8 guest. However our
cpr-transfer code also seems to be buggy as it triggers the crash -
without the cpr-transfer migration the above reproduce doesn't lead to
crash on the source VM.

I suspect that, as cpr-transfer doesn't migrate the guest memory, but
rather passes it through the memory backend object, our code might
somehow corrupt the VRAM.  However, I wasn't able to trace the
corruption so far.

Could somebody help the investigation and take a look into this?  Any
suggestions would be appreciated.  Thanks!
Possibly some memory region created by qxl is not being preserved.
Try adding these traces to see what is preserved:

-trace enable='*cpr*'
-trace enable='*ram_alloc*'
Also try adding this patch to see if it flags any ram blocks as not
compatible with cpr.  A message is printed at migration start time.
1740667681-257312-1-git-send-email-steven.sistare@oracle.com
/">https://lore.kernel.org/qemu-devel/
1740667681-257312-1-git-send-email-steven.sistare@oracle.com
/
- Steve

On 2/28/25 8:20 PM, Steven Sistare wrote:
>
On 2/28/2025 1:13 PM, Steven Sistare wrote:
>
> On 2/28/2025 12:39 PM, Andrey Drobyshev wrote:
>
>> Hi all,
>
>>
>
>> We've been experimenting with cpr-transfer migration mode recently and
>
>> have discovered the following issue with the guest QXL driver:
>
>>
>
>> Run migration source:
>
>>> EMULATOR=/path/to/emulator
>
>>> ROOTFS=/path/to/image
>
>>> QMPSOCK=/var/run/alma8qmp-src.sock
>
>>>
>
>>> $EMULATOR -enable-kvm \
>
>>>      -machine q35 \
>
>>>      -cpu host -smp 2 -m 2G \
>
>>>      -object memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/
>
>>> ram0,share=on\
>
>>>      -machine memory-backend=ram0 \
>
>>>      -machine aux-ram-share=on \
>
>>>      -drive file=$ROOTFS,media=disk,if=virtio \
>
>>>      -qmp unix:$QMPSOCK,server=on,wait=off \
>
>>>      -nographic \
>
>>>      -device qxl-vga
>
>>
>
>> Run migration target:
>
>>> EMULATOR=/path/to/emulator
>
>>> ROOTFS=/path/to/image
>
>>> QMPSOCK=/var/run/alma8qmp-dst.sock
>
>>> $EMULATOR -enable-kvm \
>
>>>      -machine q35 \
>
>>>      -cpu host -smp 2 -m 2G \
>
>>>      -object memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/
>
>>> ram0,share=on\
>
>>>      -machine memory-backend=ram0 \
>
>>>      -machine aux-ram-share=on \
>
>>>      -drive file=$ROOTFS,media=disk,if=virtio \
>
>>>      -qmp unix:$QMPSOCK,server=on,wait=off \
>
>>>      -nographic \
>
>>>      -device qxl-vga \
>
>>>      -incoming tcp:0:44444 \
>
>>>      -incoming '{"channel-type": "cpr", "addr": { "transport":
>
>>> "socket", "type": "unix", "path": "/var/run/alma8cpr-dst.sock"}}'
>
>>
>
>>
>
>> Launch the migration:
>
>>> QMPSHELL=/root/src/qemu/master/scripts/qmp/qmp-shell
>
>>> QMPSOCK=/var/run/alma8qmp-src.sock
>
>>>
>
>>> $QMPSHELL -p $QMPSOCK <<EOF
>
>>>      migrate-set-parameters mode=cpr-transfer
>
>>>      migrate channels=[{"channel-type":"main","addr":
>
>>> {"transport":"socket","type":"inet","host":"0","port":"44444"}},
>
>>> {"channel-type":"cpr","addr":
>
>>> {"transport":"socket","type":"unix","path":"/var/run/alma8cpr-
>
>>> dst.sock"}}]
>
>>> EOF
>
>>
>
>> Then, after a while, QXL guest driver on target crashes spewing the
>
>> following messages:
>
>>> [   73.962002] [TTM] Buffer eviction failed
>
>>> [   73.962072] qxl 0000:00:02.0: object_init failed for (3149824,
>
>>> 0x00000001)
>
>>> [   73.962081] [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to
>
>>> allocate VRAM BO
>
>>
>
>> That seems to be a known kernel QXL driver bug:
>
>>
>
>>
https://lore.kernel.org/all/20220907094423.93581-1-min_halo@163.com/T/
>
>>
https://lore.kernel.org/lkml/ZTgydqRlK6WX_b29@eldamar.lan/
>
>>
>
>> (the latter discussion contains that reproduce script which speeds up
>
>> the crash in the guest):
>
>>> #!/bin/bash
>
>>>
>
>>> chvt 3
>
>>>
>
>>> for j in $(seq 80); do
>
>>>          echo "$(date) starting round $j"
>
>>>          if [ "$(journalctl --boot | grep "failed to allocate VRAM
>
>>> BO")" != "" ]; then
>
>>>                  echo "bug was reproduced after $j tries"
>
>>>                  exit 1
>
>>>          fi
>
>>>          for i in $(seq 100); do
>
>>>                  dmesg > /dev/tty3
>
>>>          done
>
>>> done
>
>>>
>
>>> echo "bug could not be reproduced"
>
>>> exit 0
>
>>
>
>> The bug itself seems to remain unfixed, as I was able to reproduce that
>
>> with Fedora 41 guest, as well as AlmaLinux 8 guest. However our
>
>> cpr-transfer code also seems to be buggy as it triggers the crash -
>
>> without the cpr-transfer migration the above reproduce doesn't lead to
>
>> crash on the source VM.
>
>>
>
>> I suspect that, as cpr-transfer doesn't migrate the guest memory, but
>
>> rather passes it through the memory backend object, our code might
>
>> somehow corrupt the VRAM.  However, I wasn't able to trace the
>
>> corruption so far.
>
>>
>
>> Could somebody help the investigation and take a look into this?  Any
>
>> suggestions would be appreciated.  Thanks!
>
>
>
> Possibly some memory region created by qxl is not being preserved.
>
> Try adding these traces to see what is preserved:
>
>
>
> -trace enable='*cpr*'
>
> -trace enable='*ram_alloc*'
>
>
Also try adding this patch to see if it flags any ram blocks as not
>
compatible with cpr.  A message is printed at migration start time.
>
Â
https://lore.kernel.org/qemu-devel/1740667681-257312-1-git-send-email-
>
steven.sistare@oracle.com/
>
>
- Steve
>
With the traces enabled + the "migration: ram block cpr blockers" patch
applied:

Source:
>
cpr_find_fd pc.bios, id 0 returns -1
>
cpr_save_fd pc.bios, id 0, fd 22
>
qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 22 host
>
0x7fec18e00000
>
cpr_find_fd pc.rom, id 0 returns -1
>
cpr_save_fd pc.rom, id 0, fd 23
>
qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 23 host
>
0x7fec18c00000
>
cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns -1
>
cpr_save_fd 0000:00:01.0/e1000e.rom, id 0, fd 24
>
qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size 262144 fd
>
24 host 0x7fec18a00000
>
cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns -1
>
cpr_save_fd 0000:00:02.0/vga.vram, id 0, fd 25
>
qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size 67108864
>
fd 25 host 0x7feb77e00000
>
cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns -1
>
cpr_save_fd 0000:00:02.0/qxl.vrom, id 0, fd 27
>
qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192 fd 27
>
host 0x7fec18800000
>
cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns -1
>
cpr_save_fd 0000:00:02.0/qxl.vram, id 0, fd 28
>
qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size 67108864
>
fd 28 host 0x7feb73c00000
>
cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns -1
>
cpr_save_fd 0000:00:02.0/qxl.rom, id 0, fd 34
>
qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536 fd 34
>
host 0x7fec18600000
>
cpr_find_fd /rom@etc/acpi/tables, id 0 returns -1
>
cpr_save_fd /rom@etc/acpi/tables, id 0, fd 35
>
qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size 2097152 fd 35
>
host 0x7fec18200000
>
cpr_find_fd /rom@etc/table-loader, id 0 returns -1
>
cpr_save_fd /rom@etc/table-loader, id 0, fd 36
>
qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536 fd 36
>
host 0x7feb8b600000
>
cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns -1
>
cpr_save_fd /rom@etc/acpi/rsdp, id 0, fd 37
>
qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd 37 host
>
0x7feb8b400000
>
>
cpr_state_save cpr-transfer mode
>
cpr_transfer_output /var/run/alma8cpr-dst.sock
Target:
>
cpr_transfer_input /var/run/alma8cpr-dst.sock
>
cpr_state_load cpr-transfer mode
>
cpr_find_fd pc.bios, id 0 returns 20
>
qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 20 host
>
0x7fcdc9800000
>
cpr_find_fd pc.rom, id 0 returns 19
>
qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 19 host
>
0x7fcdc9600000
>
cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns 18
>
qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size 262144 fd
>
18 host 0x7fcdc9400000
>
cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns 17
>
qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size 67108864
>
fd 17 host 0x7fcd27e00000
>
cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns 16
>
qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192 fd 16
>
host 0x7fcdc9200000
>
cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns 15
>
qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size 67108864
>
fd 15 host 0x7fcd23c00000
>
cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns 14
>
qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536 fd 14
>
host 0x7fcdc8800000
>
cpr_find_fd /rom@etc/acpi/tables, id 0 returns 13
>
qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size 2097152 fd 13
>
host 0x7fcdc8400000
>
cpr_find_fd /rom@etc/table-loader, id 0 returns 11
>
qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536 fd 11
>
host 0x7fcdc8200000
>
cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns 10
>
qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd 10 host
>
0x7fcd3be00000
Looks like both vga.vram and qxl.vram are being preserved (with the same
addresses), and no incompatible ram blocks are found during migration.

Andrey

On 2/28/25 8:35 PM, Andrey Drobyshev wrote:
>
On 2/28/25 8:20 PM, Steven Sistare wrote:
>
> On 2/28/2025 1:13 PM, Steven Sistare wrote:
>
>> On 2/28/2025 12:39 PM, Andrey Drobyshev wrote:
>
>>> Hi all,
>
>>>
>
>>> We've been experimenting with cpr-transfer migration mode recently and
>
>>> have discovered the following issue with the guest QXL driver:
>
>>>
>
>>> Run migration source:
>
>>>> EMULATOR=/path/to/emulator
>
>>>> ROOTFS=/path/to/image
>
>>>> QMPSOCK=/var/run/alma8qmp-src.sock
>
>>>>
>
>>>> $EMULATOR -enable-kvm \
>
>>>>      -machine q35 \
>
>>>>      -cpu host -smp 2 -m 2G \
>
>>>>      -object memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/
>
>>>> ram0,share=on\
>
>>>>      -machine memory-backend=ram0 \
>
>>>>      -machine aux-ram-share=on \
>
>>>>      -drive file=$ROOTFS,media=disk,if=virtio \
>
>>>>      -qmp unix:$QMPSOCK,server=on,wait=off \
>
>>>>      -nographic \
>
>>>>      -device qxl-vga
>
>>>
>
>>> Run migration target:
>
>>>> EMULATOR=/path/to/emulator
>
>>>> ROOTFS=/path/to/image
>
>>>> QMPSOCK=/var/run/alma8qmp-dst.sock
>
>>>> $EMULATOR -enable-kvm \
>
>>>>      -machine q35 \
>
>>>>      -cpu host -smp 2 -m 2G \
>
>>>>      -object memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/
>
>>>> ram0,share=on\
>
>>>>      -machine memory-backend=ram0 \
>
>>>>      -machine aux-ram-share=on \
>
>>>>      -drive file=$ROOTFS,media=disk,if=virtio \
>
>>>>      -qmp unix:$QMPSOCK,server=on,wait=off \
>
>>>>      -nographic \
>
>>>>      -device qxl-vga \
>
>>>>      -incoming tcp:0:44444 \
>
>>>>      -incoming '{"channel-type": "cpr", "addr": { "transport":
>
>>>> "socket", "type": "unix", "path": "/var/run/alma8cpr-dst.sock"}}'
>
>>>
>
>>>
>
>>> Launch the migration:
>
>>>> QMPSHELL=/root/src/qemu/master/scripts/qmp/qmp-shell
>
>>>> QMPSOCK=/var/run/alma8qmp-src.sock
>
>>>>
>
>>>> $QMPSHELL -p $QMPSOCK <<EOF
>
>>>>      migrate-set-parameters mode=cpr-transfer
>
>>>>      migrate channels=[{"channel-type":"main","addr":
>
>>>> {"transport":"socket","type":"inet","host":"0","port":"44444"}},
>
>>>> {"channel-type":"cpr","addr":
>
>>>> {"transport":"socket","type":"unix","path":"/var/run/alma8cpr-
>
>>>> dst.sock"}}]
>
>>>> EOF
>
>>>
>
>>> Then, after a while, QXL guest driver on target crashes spewing the
>
>>> following messages:
>
>>>> [   73.962002] [TTM] Buffer eviction failed
>
>>>> [   73.962072] qxl 0000:00:02.0: object_init failed for (3149824,
>
>>>> 0x00000001)
>
>>>> [   73.962081] [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to
>
>>>> allocate VRAM BO
>
>>>
>
>>> That seems to be a known kernel QXL driver bug:
>
>>>
>
>>>
https://lore.kernel.org/all/20220907094423.93581-1-min_halo@163.com/T/
>
>>>
https://lore.kernel.org/lkml/ZTgydqRlK6WX_b29@eldamar.lan/
>
>>>
>
>>> (the latter discussion contains that reproduce script which speeds up
>
>>> the crash in the guest):
>
>>>> #!/bin/bash
>
>>>>
>
>>>> chvt 3
>
>>>>
>
>>>> for j in $(seq 80); do
>
>>>>          echo "$(date) starting round $j"
>
>>>>          if [ "$(journalctl --boot | grep "failed to allocate VRAM
>
>>>> BO")" != "" ]; then
>
>>>>                  echo "bug was reproduced after $j tries"
>
>>>>                  exit 1
>
>>>>          fi
>
>>>>          for i in $(seq 100); do
>
>>>>                  dmesg > /dev/tty3
>
>>>>          done
>
>>>> done
>
>>>>
>
>>>> echo "bug could not be reproduced"
>
>>>> exit 0
>
>>>
>
>>> The bug itself seems to remain unfixed, as I was able to reproduce that
>
>>> with Fedora 41 guest, as well as AlmaLinux 8 guest. However our
>
>>> cpr-transfer code also seems to be buggy as it triggers the crash -
>
>>> without the cpr-transfer migration the above reproduce doesn't lead to
>
>>> crash on the source VM.
>
>>>
>
>>> I suspect that, as cpr-transfer doesn't migrate the guest memory, but
>
>>> rather passes it through the memory backend object, our code might
>
>>> somehow corrupt the VRAM.  However, I wasn't able to trace the
>
>>> corruption so far.
>
>>>
>
>>> Could somebody help the investigation and take a look into this?  Any
>
>>> suggestions would be appreciated.  Thanks!
>
>>
>
>> Possibly some memory region created by qxl is not being preserved.
>
>> Try adding these traces to see what is preserved:
>
>>
>
>> -trace enable='*cpr*'
>
>> -trace enable='*ram_alloc*'
>
>
>
> Also try adding this patch to see if it flags any ram blocks as not
>
> compatible with cpr.  A message is printed at migration start time.
>
> Â
https://lore.kernel.org/qemu-devel/1740667681-257312-1-git-send-email-
>
> steven.sistare@oracle.com/
>
>
>
> - Steve
>
>
>
>
With the traces enabled + the "migration: ram block cpr blockers" patch
>
applied:
>
>
Source:
>
> cpr_find_fd pc.bios, id 0 returns -1
>
> cpr_save_fd pc.bios, id 0, fd 22
>
> qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 22 host
>
> 0x7fec18e00000
>
> cpr_find_fd pc.rom, id 0 returns -1
>
> cpr_save_fd pc.rom, id 0, fd 23
>
> qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 23 host
>
> 0x7fec18c00000
>
> cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns -1
>
> cpr_save_fd 0000:00:01.0/e1000e.rom, id 0, fd 24
>
> qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size 262144 fd
>
> 24 host 0x7fec18a00000
>
> cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns -1
>
> cpr_save_fd 0000:00:02.0/vga.vram, id 0, fd 25
>
> qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size 67108864
>
> fd 25 host 0x7feb77e00000
>
> cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns -1
>
> cpr_save_fd 0000:00:02.0/qxl.vrom, id 0, fd 27
>
> qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192 fd 27
>
> host 0x7fec18800000
>
> cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns -1
>
> cpr_save_fd 0000:00:02.0/qxl.vram, id 0, fd 28
>
> qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size 67108864
>
> fd 28 host 0x7feb73c00000
>
> cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns -1
>
> cpr_save_fd 0000:00:02.0/qxl.rom, id 0, fd 34
>
> qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536 fd 34
>
> host 0x7fec18600000
>
> cpr_find_fd /rom@etc/acpi/tables, id 0 returns -1
>
> cpr_save_fd /rom@etc/acpi/tables, id 0, fd 35
>
> qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size 2097152 fd
>
> 35 host 0x7fec18200000
>
> cpr_find_fd /rom@etc/table-loader, id 0 returns -1
>
> cpr_save_fd /rom@etc/table-loader, id 0, fd 36
>
> qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536 fd 36
>
> host 0x7feb8b600000
>
> cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns -1
>
> cpr_save_fd /rom@etc/acpi/rsdp, id 0, fd 37
>
> qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd 37 host
>
> 0x7feb8b400000
>
>
>
> cpr_state_save cpr-transfer mode
>
> cpr_transfer_output /var/run/alma8cpr-dst.sock
>
>
Target:
>
> cpr_transfer_input /var/run/alma8cpr-dst.sock
>
> cpr_state_load cpr-transfer mode
>
> cpr_find_fd pc.bios, id 0 returns 20
>
> qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 20 host
>
> 0x7fcdc9800000
>
> cpr_find_fd pc.rom, id 0 returns 19
>
> qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 19 host
>
> 0x7fcdc9600000
>
> cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns 18
>
> qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size 262144 fd
>
> 18 host 0x7fcdc9400000
>
> cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns 17
>
> qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size 67108864
>
> fd 17 host 0x7fcd27e00000
>
> cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns 16
>
> qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192 fd 16
>
> host 0x7fcdc9200000
>
> cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns 15
>
> qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size 67108864
>
> fd 15 host 0x7fcd23c00000
>
> cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns 14
>
> qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536 fd 14
>
> host 0x7fcdc8800000
>
> cpr_find_fd /rom@etc/acpi/tables, id 0 returns 13
>
> qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size 2097152 fd
>
> 13 host 0x7fcdc8400000
>
> cpr_find_fd /rom@etc/table-loader, id 0 returns 11
>
> qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536 fd 11
>
> host 0x7fcdc8200000
>
> cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns 10
>
> qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd 10 host
>
> 0x7fcd3be00000
>
>
Looks like both vga.vram and qxl.vram are being preserved (with the same
>
addresses), and no incompatible ram blocks are found during migration.
>
Sorry, addressed are not the same, of course.  However corresponding ram
blocks do seem to be preserved and initialized.

On 2/28/2025 1:37 PM, Andrey Drobyshev wrote:
On 2/28/25 8:35 PM, Andrey Drobyshev wrote:
On 2/28/25 8:20 PM, Steven Sistare wrote:
On 2/28/2025 1:13 PM, Steven Sistare wrote:
On 2/28/2025 12:39 PM, Andrey Drobyshev wrote:
Hi all,

We've been experimenting with cpr-transfer migration mode recently and
have discovered the following issue with the guest QXL driver:

Run migration source:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-src.sock

$EMULATOR -enable-kvm \
      -machine q35 \
      -cpu host -smp 2 -m 2G \
      -object memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/
ram0,share=on\
      -machine memory-backend=ram0 \
      -machine aux-ram-share=on \
      -drive file=$ROOTFS,media=disk,if=virtio \
      -qmp unix:$QMPSOCK,server=on,wait=off \
      -nographic \
      -device qxl-vga
Run migration target:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-dst.sock
$EMULATOR -enable-kvm \
      -machine q35 \
      -cpu host -smp 2 -m 2G \
      -object memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/
ram0,share=on\
      -machine memory-backend=ram0 \
      -machine aux-ram-share=on \
      -drive file=$ROOTFS,media=disk,if=virtio \
      -qmp unix:$QMPSOCK,server=on,wait=off \
      -nographic \
      -device qxl-vga \
      -incoming tcp:0:44444 \
      -incoming '{"channel-type": "cpr", "addr": { "transport":
"socket", "type": "unix", "path": "/var/run/alma8cpr-dst.sock"}}'
Launch the migration:
QMPSHELL=/root/src/qemu/master/scripts/qmp/qmp-shell
QMPSOCK=/var/run/alma8qmp-src.sock

$QMPSHELL -p $QMPSOCK <<EOF
      migrate-set-parameters mode=cpr-transfer
      migrate channels=[{"channel-type":"main","addr":
{"transport":"socket","type":"inet","host":"0","port":"44444"}},
{"channel-type":"cpr","addr":
{"transport":"socket","type":"unix","path":"/var/run/alma8cpr-
dst.sock"}}]
EOF
Then, after a while, QXL guest driver on target crashes spewing the
following messages:
[   73.962002] [TTM] Buffer eviction failed
[   73.962072] qxl 0000:00:02.0: object_init failed for (3149824,
0x00000001)
[   73.962081] [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to
allocate VRAM BO
That seems to be a known kernel QXL driver bug:
https://lore.kernel.org/all/20220907094423.93581-1-min_halo@163.com/T/
https://lore.kernel.org/lkml/ZTgydqRlK6WX_b29@eldamar.lan/
(the latter discussion contains that reproduce script which speeds up
the crash in the guest):
#!/bin/bash

chvt 3

for j in $(seq 80); do
          echo "$(date) starting round $j"
          if [ "$(journalctl --boot | grep "failed to allocate VRAM
BO")" != "" ]; then
                  echo "bug was reproduced after $j tries"
                  exit 1
          fi
          for i in $(seq 100); do
                  dmesg > /dev/tty3
          done
done

echo "bug could not be reproduced"
exit 0
The bug itself seems to remain unfixed, as I was able to reproduce that
with Fedora 41 guest, as well as AlmaLinux 8 guest. However our
cpr-transfer code also seems to be buggy as it triggers the crash -
without the cpr-transfer migration the above reproduce doesn't lead to
crash on the source VM.

I suspect that, as cpr-transfer doesn't migrate the guest memory, but
rather passes it through the memory backend object, our code might
somehow corrupt the VRAM.  However, I wasn't able to trace the
corruption so far.

Could somebody help the investigation and take a look into this?  Any
suggestions would be appreciated.  Thanks!
Possibly some memory region created by qxl is not being preserved.
Try adding these traces to see what is preserved:

-trace enable='*cpr*'
-trace enable='*ram_alloc*'
Also try adding this patch to see if it flags any ram blocks as not
compatible with cpr.  A message is printed at migration start time.
 Â
https://lore.kernel.org/qemu-devel/1740667681-257312-1-git-send-email-
steven.sistare@oracle.com/

- Steve
With the traces enabled + the "migration: ram block cpr blockers" patch
applied:

Source:
cpr_find_fd pc.bios, id 0 returns -1
cpr_save_fd pc.bios, id 0, fd 22
qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 22 host 
0x7fec18e00000
cpr_find_fd pc.rom, id 0 returns -1
cpr_save_fd pc.rom, id 0, fd 23
qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 23 host 
0x7fec18c00000
cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns -1
cpr_save_fd 0000:00:01.0/e1000e.rom, id 0, fd 24
qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size 262144 fd 24 
host 0x7fec18a00000
cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns -1
cpr_save_fd 0000:00:02.0/vga.vram, id 0, fd 25
qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size 67108864 fd 
25 host 0x7feb77e00000
cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.vrom, id 0, fd 27
qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192 fd 27 host 
0x7fec18800000
cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.vram, id 0, fd 28
qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size 67108864 fd 
28 host 0x7feb73c00000
cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.rom, id 0, fd 34
qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536 fd 34 host 
0x7fec18600000
cpr_find_fd /rom@etc/acpi/tables, id 0 returns -1
cpr_save_fd /rom@etc/acpi/tables, id 0, fd 35
qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size 2097152 fd 35 
host 0x7fec18200000
cpr_find_fd /rom@etc/table-loader, id 0 returns -1
cpr_save_fd /rom@etc/table-loader, id 0, fd 36
qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536 fd 36 host 
0x7feb8b600000
cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns -1
cpr_save_fd /rom@etc/acpi/rsdp, id 0, fd 37
qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd 37 host 
0x7feb8b400000

cpr_state_save cpr-transfer mode
cpr_transfer_output /var/run/alma8cpr-dst.sock
Target:
cpr_transfer_input /var/run/alma8cpr-dst.sock
cpr_state_load cpr-transfer mode
cpr_find_fd pc.bios, id 0 returns 20
qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 20 host 
0x7fcdc9800000
cpr_find_fd pc.rom, id 0 returns 19
qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 19 host 
0x7fcdc9600000
cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns 18
qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size 262144 fd 18 
host 0x7fcdc9400000
cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns 17
qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size 67108864 fd 
17 host 0x7fcd27e00000
cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns 16
qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192 fd 16 host 
0x7fcdc9200000
cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns 15
qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size 67108864 fd 
15 host 0x7fcd23c00000
cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns 14
qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536 fd 14 host 
0x7fcdc8800000
cpr_find_fd /rom@etc/acpi/tables, id 0 returns 13
qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size 2097152 fd 13 
host 0x7fcdc8400000
cpr_find_fd /rom@etc/table-loader, id 0 returns 11
qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536 fd 11 host 
0x7fcdc8200000
cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns 10
qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd 10 host 
0x7fcd3be00000
Looks like both vga.vram and qxl.vram are being preserved (with the same
addresses), and no incompatible ram blocks are found during migration.
Sorry, addressed are not the same, of course.  However corresponding ram
blocks do seem to be preserved and initialized.
So far, I have not reproduced the guest driver failure.

However, I have isolated places where new QEMU improperly writes to
the qxl memory regions prior to starting the guest, by mmap'ing them
readonly after cpr:

  qemu_ram_alloc_internal()
    if (reused && (strstr(name, "qxl") || strstr("name", "vga")))
        ram_flags |= RAM_READONLY;
    new_block = qemu_ram_alloc_from_fd(...)

I have attached a draft fix; try it and let me know.
My console window looks fine before and after cpr, using
-vnc $hostip:0 -vga qxl

- Steve
0001-hw-qxl-cpr-support-preliminary.patch
Description:
Text document

On 3/4/25 9:05 PM, Steven Sistare wrote:
>
On 2/28/2025 1:37 PM, Andrey Drobyshev wrote:
>
> On 2/28/25 8:35 PM, Andrey Drobyshev wrote:
>
>> On 2/28/25 8:20 PM, Steven Sistare wrote:
>
>>> On 2/28/2025 1:13 PM, Steven Sistare wrote:
>
>>>> On 2/28/2025 12:39 PM, Andrey Drobyshev wrote:
>
>>>>> Hi all,
>
>>>>>
>
>>>>> We've been experimenting with cpr-transfer migration mode recently
>
>>>>> and
>
>>>>> have discovered the following issue with the guest QXL driver:
>
>>>>>
>
>>>>> Run migration source:
>
>>>>>> EMULATOR=/path/to/emulator
>
>>>>>> ROOTFS=/path/to/image
>
>>>>>> QMPSOCK=/var/run/alma8qmp-src.sock
>
>>>>>>
>
>>>>>> $EMULATOR -enable-kvm \
>
>>>>>>       -machine q35 \
>
>>>>>>       -cpu host -smp 2 -m 2G \
>
>>>>>>       -object memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/
>
>>>>>> ram0,share=on\
>
>>>>>>       -machine memory-backend=ram0 \
>
>>>>>>       -machine aux-ram-share=on \
>
>>>>>>       -drive file=$ROOTFS,media=disk,if=virtio \
>
>>>>>>       -qmp unix:$QMPSOCK,server=on,wait=off \
>
>>>>>>       -nographic \
>
>>>>>>       -device qxl-vga
>
>>>>>
>
>>>>> Run migration target:
>
>>>>>> EMULATOR=/path/to/emulator
>
>>>>>> ROOTFS=/path/to/image
>
>>>>>> QMPSOCK=/var/run/alma8qmp-dst.sock
>
>>>>>> $EMULATOR -enable-kvm \
>
>>>>>>       -machine q35 \
>
>>>>>>       -cpu host -smp 2 -m 2G \
>
>>>>>>       -object memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/
>
>>>>>> ram0,share=on\
>
>>>>>>       -machine memory-backend=ram0 \
>
>>>>>>       -machine aux-ram-share=on \
>
>>>>>>       -drive file=$ROOTFS,media=disk,if=virtio \
>
>>>>>>       -qmp unix:$QMPSOCK,server=on,wait=off \
>
>>>>>>       -nographic \
>
>>>>>>       -device qxl-vga \
>
>>>>>>       -incoming tcp:0:44444 \
>
>>>>>>       -incoming '{"channel-type": "cpr", "addr": { "transport":
>
>>>>>> "socket", "type": "unix", "path": "/var/run/alma8cpr-dst.sock"}}'
>
>>>>>
>
>>>>>
>
>>>>> Launch the migration:
>
>>>>>> QMPSHELL=/root/src/qemu/master/scripts/qmp/qmp-shell
>
>>>>>> QMPSOCK=/var/run/alma8qmp-src.sock
>
>>>>>>
>
>>>>>> $QMPSHELL -p $QMPSOCK <<EOF
>
>>>>>>       migrate-set-parameters mode=cpr-transfer
>
>>>>>>       migrate channels=[{"channel-type":"main","addr":
>
>>>>>> {"transport":"socket","type":"inet","host":"0","port":"44444"}},
>
>>>>>> {"channel-type":"cpr","addr":
>
>>>>>> {"transport":"socket","type":"unix","path":"/var/run/alma8cpr-
>
>>>>>> dst.sock"}}]
>
>>>>>> EOF
>
>>>>>
>
>>>>> Then, after a while, QXL guest driver on target crashes spewing the
>
>>>>> following messages:
>
>>>>>> [   73.962002] [TTM] Buffer eviction failed
>
>>>>>> [   73.962072] qxl 0000:00:02.0: object_init failed for (3149824,
>
>>>>>> 0x00000001)
>
>>>>>> [   73.962081] [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to
>
>>>>>> allocate VRAM BO
>
>>>>>
>
>>>>> That seems to be a known kernel QXL driver bug:
>
>>>>>
>
>>>>>
https://lore.kernel.org/all/20220907094423.93581-1-
>
>>>>> min_halo@163.com/T/
>
>>>>>
https://lore.kernel.org/lkml/ZTgydqRlK6WX_b29@eldamar.lan/
>
>>>>>
>
>>>>> (the latter discussion contains that reproduce script which speeds up
>
>>>>> the crash in the guest):
>
>>>>>> #!/bin/bash
>
>>>>>>
>
>>>>>> chvt 3
>
>>>>>>
>
>>>>>> for j in $(seq 80); do
>
>>>>>>           echo "$(date) starting round $j"
>
>>>>>>           if [ "$(journalctl --boot | grep "failed to allocate VRAM
>
>>>>>> BO")" != "" ]; then
>
>>>>>>                   echo "bug was reproduced after $j tries"
>
>>>>>>                   exit 1
>
>>>>>>           fi
>
>>>>>>           for i in $(seq 100); do
>
>>>>>>                   dmesg > /dev/tty3
>
>>>>>>           done
>
>>>>>> done
>
>>>>>>
>
>>>>>> echo "bug could not be reproduced"
>
>>>>>> exit 0
>
>>>>>
>
>>>>> The bug itself seems to remain unfixed, as I was able to reproduce
>
>>>>> that
>
>>>>> with Fedora 41 guest, as well as AlmaLinux 8 guest. However our
>
>>>>> cpr-transfer code also seems to be buggy as it triggers the crash -
>
>>>>> without the cpr-transfer migration the above reproduce doesn't
>
>>>>> lead to
>
>>>>> crash on the source VM.
>
>>>>>
>
>>>>> I suspect that, as cpr-transfer doesn't migrate the guest memory, but
>
>>>>> rather passes it through the memory backend object, our code might
>
>>>>> somehow corrupt the VRAM.  However, I wasn't able to trace the
>
>>>>> corruption so far.
>
>>>>>
>
>>>>> Could somebody help the investigation and take a look into this?  Any
>
>>>>> suggestions would be appreciated.  Thanks!
>
>>>>
>
>>>> Possibly some memory region created by qxl is not being preserved.
>
>>>> Try adding these traces to see what is preserved:
>
>>>>
>
>>>> -trace enable='*cpr*'
>
>>>> -trace enable='*ram_alloc*'
>
>>>
>
>>> Also try adding this patch to see if it flags any ram blocks as not
>
>>> compatible with cpr.  A message is printed at migration start time.
>
>>>  Â
https://lore.kernel.org/qemu-devel/1740667681-257312-1-git-send-
>
>>> email-
>
>>> steven.sistare@oracle.com/
>
>>>
>
>>> - Steve
>
>>>
>
>>
>
>> With the traces enabled + the "migration: ram block cpr blockers" patch
>
>> applied:
>
>>
>
>> Source:
>
>>> cpr_find_fd pc.bios, id 0 returns -1
>
>>> cpr_save_fd pc.bios, id 0, fd 22
>
>>> qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 22 host
>
>>> 0x7fec18e00000
>
>>> cpr_find_fd pc.rom, id 0 returns -1
>
>>> cpr_save_fd pc.rom, id 0, fd 23
>
>>> qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 23 host
>
>>> 0x7fec18c00000
>
>>> cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns -1
>
>>> cpr_save_fd 0000:00:01.0/e1000e.rom, id 0, fd 24
>
>>> qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size
>
>>> 262144 fd 24 host 0x7fec18a00000
>
>>> cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns -1
>
>>> cpr_save_fd 0000:00:02.0/vga.vram, id 0, fd 25
>
>>> qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size
>
>>> 67108864 fd 25 host 0x7feb77e00000
>
>>> cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns -1
>
>>> cpr_save_fd 0000:00:02.0/qxl.vrom, id 0, fd 27
>
>>> qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192
>
>>> fd 27 host 0x7fec18800000
>
>>> cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns -1
>
>>> cpr_save_fd 0000:00:02.0/qxl.vram, id 0, fd 28
>
>>> qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size
>
>>> 67108864 fd 28 host 0x7feb73c00000
>
>>> cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns -1
>
>>> cpr_save_fd 0000:00:02.0/qxl.rom, id 0, fd 34
>
>>> qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536
>
>>> fd 34 host 0x7fec18600000
>
>>> cpr_find_fd /rom@etc/acpi/tables, id 0 returns -1
>
>>> cpr_save_fd /rom@etc/acpi/tables, id 0, fd 35
>
>>> qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size
>
>>> 2097152 fd 35 host 0x7fec18200000
>
>>> cpr_find_fd /rom@etc/table-loader, id 0 returns -1
>
>>> cpr_save_fd /rom@etc/table-loader, id 0, fd 36
>
>>> qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536
>
>>> fd 36 host 0x7feb8b600000
>
>>> cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns -1
>
>>> cpr_save_fd /rom@etc/acpi/rsdp, id 0, fd 37
>
>>> qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd
>
>>> 37 host 0x7feb8b400000
>
>>>
>
>>> cpr_state_save cpr-transfer mode
>
>>> cpr_transfer_output /var/run/alma8cpr-dst.sock
>
>>
>
>> Target:
>
>>> cpr_transfer_input /var/run/alma8cpr-dst.sock
>
>>> cpr_state_load cpr-transfer mode
>
>>> cpr_find_fd pc.bios, id 0 returns 20
>
>>> qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 20 host
>
>>> 0x7fcdc9800000
>
>>> cpr_find_fd pc.rom, id 0 returns 19
>
>>> qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 19 host
>
>>> 0x7fcdc9600000
>
>>> cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns 18
>
>>> qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size
>
>>> 262144 fd 18 host 0x7fcdc9400000
>
>>> cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns 17
>
>>> qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size
>
>>> 67108864 fd 17 host 0x7fcd27e00000
>
>>> cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns 16
>
>>> qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192
>
>>> fd 16 host 0x7fcdc9200000
>
>>> cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns 15
>
>>> qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size
>
>>> 67108864 fd 15 host 0x7fcd23c00000
>
>>> cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns 14
>
>>> qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536
>
>>> fd 14 host 0x7fcdc8800000
>
>>> cpr_find_fd /rom@etc/acpi/tables, id 0 returns 13
>
>>> qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size
>
>>> 2097152 fd 13 host 0x7fcdc8400000
>
>>> cpr_find_fd /rom@etc/table-loader, id 0 returns 11
>
>>> qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536
>
>>> fd 11 host 0x7fcdc8200000
>
>>> cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns 10
>
>>> qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd
>
>>> 10 host 0x7fcd3be00000
>
>>
>
>> Looks like both vga.vram and qxl.vram are being preserved (with the same
>
>> addresses), and no incompatible ram blocks are found during migration.
>
>
>
> Sorry, addressed are not the same, of course.  However corresponding ram
>
> blocks do seem to be preserved and initialized.
>
>
So far, I have not reproduced the guest driver failure.
>
>
However, I have isolated places where new QEMU improperly writes to
>
the qxl memory regions prior to starting the guest, by mmap'ing them
>
readonly after cpr:
>
>
  qemu_ram_alloc_internal()
>
    if (reused && (strstr(name, "qxl") || strstr("name", "vga")))
>
        ram_flags |= RAM_READONLY;
>
    new_block = qemu_ram_alloc_from_fd(...)
>
>
I have attached a draft fix; try it and let me know.
>
My console window looks fine before and after cpr, using
>
-vnc $hostip:0 -vga qxl
>
>
- Steve
Regarding the reproduce: when I launch the buggy version with the same
options as you, i.e. "-vnc 0.0.0.0:$port -vga qxl", and do cpr-transfer,
my VNC client silently hangs on the target after a while.  Could it
happen on your stand as well?  Could you try launching VM with
"-nographic -device qxl-vga"?  That way VM's serial console is given you
directly in the shell, so when qxl driver crashes you're still able to
inspect the kernel messages.

As for your patch, I can report that it doesn't resolve the issue as it
is.  But I was able to track down another possible memory corruption
using your approach with readonly mmap'ing:

>
Program terminated with signal SIGSEGV, Segmentation fault.
>
#0  init_qxl_ram (d=0x5638996e0e70) at ../hw/display/qxl.c:412
>
412         d->ram->magic       = cpu_to_le32(QXL_RAM_MAGIC);
>
[Current thread is 1 (Thread 0x7f1a4f83b480 (LWP 229798))]
>
(gdb) bt
>
#0  init_qxl_ram (d=0x5638996e0e70) at ../hw/display/qxl.c:412
>
#1  0x0000563896e7f467 in qxl_realize_common (qxl=0x5638996e0e70,
>
errp=0x7ffd3c2b8170) at ../hw/display/qxl.c:2142
>
#2  0x0000563896e7fda1 in qxl_realize_primary (dev=0x5638996e0e70,
>
errp=0x7ffd3c2b81d0) at ../hw/display/qxl.c:2257
>
#3  0x0000563896c7e8f2 in pci_qdev_realize (qdev=0x5638996e0e70,
>
errp=0x7ffd3c2b8250) at ../hw/pci/pci.c:2174
>
#4  0x00005638970eb54b in device_set_realized (obj=0x5638996e0e70,
>
value=true, errp=0x7ffd3c2b84e0) at ../hw/core/qdev.c:494
>
#5  0x00005638970f5e14 in property_set_bool (obj=0x5638996e0e70,
>
v=0x5638996f3770, name=0x56389759b141 "realized", opaque=0x5638987893d0,
>
errp=0x7ffd3c2b84e0)
>
at ../qom/object.c:2374
>
#6  0x00005638970f39f8 in object_property_set (obj=0x5638996e0e70,
>
name=0x56389759b141 "realized", v=0x5638996f3770, errp=0x7ffd3c2b84e0)
>
at ../qom/object.c:1449
>
#7  0x00005638970f8586 in object_property_set_qobject (obj=0x5638996e0e70,
>
name=0x56389759b141 "realized", value=0x5638996df900, errp=0x7ffd3c2b84e0)
>
at ../qom/qom-qobject.c:28
>
#8  0x00005638970f3d8d in object_property_set_bool (obj=0x5638996e0e70,
>
name=0x56389759b141 "realized", value=true, errp=0x7ffd3c2b84e0)
>
at ../qom/object.c:1519
>
#9  0x00005638970eacb0 in qdev_realize (dev=0x5638996e0e70,
>
bus=0x563898cf3c20, errp=0x7ffd3c2b84e0) at ../hw/core/qdev.c:276
>
#10 0x0000563896dba675 in qdev_device_add_from_qdict (opts=0x5638996dfe50,
>
from_json=false, errp=0x7ffd3c2b84e0) at ../system/qdev-monitor.c:714
>
#11 0x0000563896dba721 in qdev_device_add (opts=0x563898786150,
>
errp=0x56389855dc40 <error_fatal>) at ../system/qdev-monitor.c:733
>
#12 0x0000563896dc48f1 in device_init_func (opaque=0x0, opts=0x563898786150,
>
errp=0x56389855dc40 <error_fatal>) at ../system/vl.c:1207
>
#13 0x000056389737a6cc in qemu_opts_foreach
>
(list=0x563898427b60 <qemu_device_opts>, func=0x563896dc48ca
>
<device_init_func>, opaque=0x0, errp=0x56389855dc40 <error_fatal>)
>
at ../util/qemu-option.c:1135
>
#14 0x0000563896dc89b5 in qemu_create_cli_devices () at ../system/vl.c:2745
>
#15 0x0000563896dc8c00 in qmp_x_exit_preconfig (errp=0x56389855dc40
>
<error_fatal>) at ../system/vl.c:2806
>
#16 0x0000563896dcb5de in qemu_init (argc=33, argv=0x7ffd3c2b8948) at
>
../system/vl.c:3838
>
#17 0x0000563897297323 in main (argc=33, argv=0x7ffd3c2b8948) at
>
../system/main.c:72
So the attached adjusted version of your patch does seem to help.  At
least I can't reproduce the crash on my stand.

I'm wondering, could it be useful to explicitly mark all the reused
memory regions readonly upon cpr-transfer, and then make them writable
back again after the migration is done?  That way we will be segfaulting
early on instead of debugging tricky memory corruptions.

Andrey
0001-hw-qxl-cpr-support-preliminary.patch
Description:
Text Data

On 3/5/2025 11:50 AM, Andrey Drobyshev wrote:
On 3/4/25 9:05 PM, Steven Sistare wrote:
On 2/28/2025 1:37 PM, Andrey Drobyshev wrote:
On 2/28/25 8:35 PM, Andrey Drobyshev wrote:
On 2/28/25 8:20 PM, Steven Sistare wrote:
On 2/28/2025 1:13 PM, Steven Sistare wrote:
On 2/28/2025 12:39 PM, Andrey Drobyshev wrote:
Hi all,

We've been experimenting with cpr-transfer migration mode recently
and
have discovered the following issue with the guest QXL driver:

Run migration source:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-src.sock

$EMULATOR -enable-kvm \
       -machine q35 \
       -cpu host -smp 2 -m 2G \
       -object memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/
ram0,share=on\
       -machine memory-backend=ram0 \
       -machine aux-ram-share=on \
       -drive file=$ROOTFS,media=disk,if=virtio \
       -qmp unix:$QMPSOCK,server=on,wait=off \
       -nographic \
       -device qxl-vga
Run migration target:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-dst.sock
$EMULATOR -enable-kvm \
       -machine q35 \
       -cpu host -smp 2 -m 2G \
       -object memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/
ram0,share=on\
       -machine memory-backend=ram0 \
       -machine aux-ram-share=on \
       -drive file=$ROOTFS,media=disk,if=virtio \
       -qmp unix:$QMPSOCK,server=on,wait=off \
       -nographic \
       -device qxl-vga \
       -incoming tcp:0:44444 \
       -incoming '{"channel-type": "cpr", "addr": { "transport":
"socket", "type": "unix", "path": "/var/run/alma8cpr-dst.sock"}}'
Launch the migration:
QMPSHELL=/root/src/qemu/master/scripts/qmp/qmp-shell
QMPSOCK=/var/run/alma8qmp-src.sock

$QMPSHELL -p $QMPSOCK <<EOF
       migrate-set-parameters mode=cpr-transfer
       migrate channels=[{"channel-type":"main","addr":
{"transport":"socket","type":"inet","host":"0","port":"44444"}},
{"channel-type":"cpr","addr":
{"transport":"socket","type":"unix","path":"/var/run/alma8cpr-
dst.sock"}}]
EOF
Then, after a while, QXL guest driver on target crashes spewing the
following messages:
[   73.962002] [TTM] Buffer eviction failed
[   73.962072] qxl 0000:00:02.0: object_init failed for (3149824,
0x00000001)
[   73.962081] [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to
allocate VRAM BO
That seems to be a known kernel QXL driver bug:
https://lore.kernel.org/all/20220907094423.93581-1-
min_halo@163.com/T/
https://lore.kernel.org/lkml/ZTgydqRlK6WX_b29@eldamar.lan/
(the latter discussion contains that reproduce script which speeds up
the crash in the guest):
#!/bin/bash

chvt 3

for j in $(seq 80); do
           echo "$(date) starting round $j"
           if [ "$(journalctl --boot | grep "failed to allocate VRAM
BO")" != "" ]; then
                   echo "bug was reproduced after $j tries"
                   exit 1
           fi
           for i in $(seq 100); do
                   dmesg > /dev/tty3
           done
done

echo "bug could not be reproduced"
exit 0
The bug itself seems to remain unfixed, as I was able to reproduce
that
with Fedora 41 guest, as well as AlmaLinux 8 guest. However our
cpr-transfer code also seems to be buggy as it triggers the crash -
without the cpr-transfer migration the above reproduce doesn't
lead to
crash on the source VM.

I suspect that, as cpr-transfer doesn't migrate the guest memory, but
rather passes it through the memory backend object, our code might
somehow corrupt the VRAM.  However, I wasn't able to trace the
corruption so far.

Could somebody help the investigation and take a look into this?  Any
suggestions would be appreciated.  Thanks!
Possibly some memory region created by qxl is not being preserved.
Try adding these traces to see what is preserved:

-trace enable='*cpr*'
-trace enable='*ram_alloc*'
Also try adding this patch to see if it flags any ram blocks as not
compatible with cpr.  A message is printed at migration start time.
  Â
https://lore.kernel.org/qemu-devel/1740667681-257312-1-git-send-
email-
steven.sistare@oracle.com/

- Steve
With the traces enabled + the "migration: ram block cpr blockers" patch
applied:

Source:
cpr_find_fd pc.bios, id 0 returns -1
cpr_save_fd pc.bios, id 0, fd 22
qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 22 host
0x7fec18e00000
cpr_find_fd pc.rom, id 0 returns -1
cpr_save_fd pc.rom, id 0, fd 23
qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 23 host
0x7fec18c00000
cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns -1
cpr_save_fd 0000:00:01.0/e1000e.rom, id 0, fd 24
qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size
262144 fd 24 host 0x7fec18a00000
cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns -1
cpr_save_fd 0000:00:02.0/vga.vram, id 0, fd 25
qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size
67108864 fd 25 host 0x7feb77e00000
cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.vrom, id 0, fd 27
qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192
fd 27 host 0x7fec18800000
cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.vram, id 0, fd 28
qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size
67108864 fd 28 host 0x7feb73c00000
cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.rom, id 0, fd 34
qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536
fd 34 host 0x7fec18600000
cpr_find_fd /rom@etc/acpi/tables, id 0 returns -1
cpr_save_fd /rom@etc/acpi/tables, id 0, fd 35
qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size
2097152 fd 35 host 0x7fec18200000
cpr_find_fd /rom@etc/table-loader, id 0 returns -1
cpr_save_fd /rom@etc/table-loader, id 0, fd 36
qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536
fd 36 host 0x7feb8b600000
cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns -1
cpr_save_fd /rom@etc/acpi/rsdp, id 0, fd 37
qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd
37 host 0x7feb8b400000

cpr_state_save cpr-transfer mode
cpr_transfer_output /var/run/alma8cpr-dst.sock
Target:
cpr_transfer_input /var/run/alma8cpr-dst.sock
cpr_state_load cpr-transfer mode
cpr_find_fd pc.bios, id 0 returns 20
qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 20 host
0x7fcdc9800000
cpr_find_fd pc.rom, id 0 returns 19
qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 19 host
0x7fcdc9600000
cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns 18
qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size
262144 fd 18 host 0x7fcdc9400000
cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns 17
qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size
67108864 fd 17 host 0x7fcd27e00000
cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns 16
qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192
fd 16 host 0x7fcdc9200000
cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns 15
qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size
67108864 fd 15 host 0x7fcd23c00000
cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns 14
qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536
fd 14 host 0x7fcdc8800000
cpr_find_fd /rom@etc/acpi/tables, id 0 returns 13
qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size
2097152 fd 13 host 0x7fcdc8400000
cpr_find_fd /rom@etc/table-loader, id 0 returns 11
qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536
fd 11 host 0x7fcdc8200000
cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns 10
qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd
10 host 0x7fcd3be00000
Looks like both vga.vram and qxl.vram are being preserved (with the same
addresses), and no incompatible ram blocks are found during migration.
Sorry, addressed are not the same, of course.  However corresponding ram
blocks do seem to be preserved and initialized.
So far, I have not reproduced the guest driver failure.

However, I have isolated places where new QEMU improperly writes to
the qxl memory regions prior to starting the guest, by mmap'ing them
readonly after cpr:

   qemu_ram_alloc_internal()
     if (reused && (strstr(name, "qxl") || strstr("name", "vga")))
         ram_flags |= RAM_READONLY;
     new_block = qemu_ram_alloc_from_fd(...)

I have attached a draft fix; try it and let me know.
My console window looks fine before and after cpr, using
-vnc $hostip:0 -vga qxl

- Steve
Regarding the reproduce: when I launch the buggy version with the same
options as you, i.e. "-vnc 0.0.0.0:$port -vga qxl", and do cpr-transfer,
my VNC client silently hangs on the target after a while.  Could it
happen on your stand as well?
cpr does not preserve the vnc connection and session.  To test, I specify
port 0 for the source VM and port 1 for the dest.  When the src vnc goes
dormant the dest vnc becomes active.
Could you try launching VM with
"-nographic -device qxl-vga"?  That way VM's serial console is given you
directly in the shell, so when qxl driver crashes you're still able to
inspect the kernel messages.
I have been running like that, but have not reproduced the qxl driver crash,
and I suspect my guest image+kernel is too old.  However, once I realized the
issue was post-cpr modification of qxl memory, I switched my attention to the
fix.
As for your patch, I can report that it doesn't resolve the issue as it
is.  But I was able to track down another possible memory corruption
using your approach with readonly mmap'ing:
Program terminated with signal SIGSEGV, Segmentation fault.
#0  init_qxl_ram (d=0x5638996e0e70) at ../hw/display/qxl.c:412
412         d->ram->magic       = cpu_to_le32(QXL_RAM_MAGIC);
[Current thread is 1 (Thread 0x7f1a4f83b480 (LWP 229798))]
(gdb) bt
#0  init_qxl_ram (d=0x5638996e0e70) at ../hw/display/qxl.c:412
#1  0x0000563896e7f467 in qxl_realize_common (qxl=0x5638996e0e70, 
errp=0x7ffd3c2b8170) at ../hw/display/qxl.c:2142
#2  0x0000563896e7fda1 in qxl_realize_primary (dev=0x5638996e0e70, 
errp=0x7ffd3c2b81d0) at ../hw/display/qxl.c:2257
#3  0x0000563896c7e8f2 in pci_qdev_realize (qdev=0x5638996e0e70, 
errp=0x7ffd3c2b8250) at ../hw/pci/pci.c:2174
#4  0x00005638970eb54b in device_set_realized (obj=0x5638996e0e70, value=true, 
errp=0x7ffd3c2b84e0) at ../hw/core/qdev.c:494
#5  0x00005638970f5e14 in property_set_bool (obj=0x5638996e0e70, v=0x5638996f3770, 
name=0x56389759b141 "realized", opaque=0x5638987893d0, errp=0x7ffd3c2b84e0)
     at ../qom/object.c:2374
#6  0x00005638970f39f8 in object_property_set (obj=0x5638996e0e70, name=0x56389759b141 
"realized", v=0x5638996f3770, errp=0x7ffd3c2b84e0)
     at ../qom/object.c:1449
#7  0x00005638970f8586 in object_property_set_qobject (obj=0x5638996e0e70, 
name=0x56389759b141 "realized", value=0x5638996df900, errp=0x7ffd3c2b84e0)
     at ../qom/qom-qobject.c:28
#8  0x00005638970f3d8d in object_property_set_bool (obj=0x5638996e0e70, 
name=0x56389759b141 "realized", value=true, errp=0x7ffd3c2b84e0)
     at ../qom/object.c:1519
#9  0x00005638970eacb0 in qdev_realize (dev=0x5638996e0e70, bus=0x563898cf3c20, 
errp=0x7ffd3c2b84e0) at ../hw/core/qdev.c:276
#10 0x0000563896dba675 in qdev_device_add_from_qdict (opts=0x5638996dfe50, 
from_json=false, errp=0x7ffd3c2b84e0) at ../system/qdev-monitor.c:714
#11 0x0000563896dba721 in qdev_device_add (opts=0x563898786150, errp=0x56389855dc40 
<error_fatal>) at ../system/qdev-monitor.c:733
#12 0x0000563896dc48f1 in device_init_func (opaque=0x0, opts=0x563898786150, 
errp=0x56389855dc40 <error_fatal>) at ../system/vl.c:1207
#13 0x000056389737a6cc in qemu_opts_foreach
     (list=0x563898427b60 <qemu_device_opts>, func=0x563896dc48ca <device_init_func>, 
opaque=0x0, errp=0x56389855dc40 <error_fatal>)
     at ../util/qemu-option.c:1135
#14 0x0000563896dc89b5 in qemu_create_cli_devices () at ../system/vl.c:2745
#15 0x0000563896dc8c00 in qmp_x_exit_preconfig (errp=0x56389855dc40 
<error_fatal>) at ../system/vl.c:2806
#16 0x0000563896dcb5de in qemu_init (argc=33, argv=0x7ffd3c2b8948) at 
../system/vl.c:3838
#17 0x0000563897297323 in main (argc=33, argv=0x7ffd3c2b8948) at 
../system/main.c:72
So the attached adjusted version of your patch does seem to help.  At
least I can't reproduce the crash on my stand.
Thanks for the stack trace; the calls to SPICE_RING_INIT in init_qxl_ram are
definitely harmful.  Try V2 of the patch, attached, which skips the lines
of init_qxl_ram that modify guest memory.
I'm wondering, could it be useful to explicitly mark all the reused
memory regions readonly upon cpr-transfer, and then make them writable
back again after the migration is done?  That way we will be segfaulting
early on instead of debugging tricky memory corruptions.
It's a useful debugging technique, but changing protection on a large memory 
region
can be too expensive for production due to TLB shootdowns.

Also, there are cases where writes are performed but the value is guaranteed to
be the same:
  qxl_post_load()
    qxl_set_mode()
      d->rom->mode = cpu_to_le32(modenr);
The value is the same because mode and shadow_rom.mode were passed in vmstate
from old qemu.

- Steve
0001-hw-qxl-cpr-support-preliminary-V2.patch
Description:
Text document

On 3/5/25 22:19, Steven Sistare wrote:
On 3/5/2025 11:50 AM, Andrey Drobyshev wrote:
On 3/4/25 9:05 PM, Steven Sistare wrote:
On 2/28/2025 1:37 PM, Andrey Drobyshev wrote:
On 2/28/25 8:35 PM, Andrey Drobyshev wrote:
On 2/28/25 8:20 PM, Steven Sistare wrote:
On 2/28/2025 1:13 PM, Steven Sistare wrote:
On 2/28/2025 12:39 PM, Andrey Drobyshev wrote:
Hi all,

We've been experimenting with cpr-transfer migration mode recently
and
have discovered the following issue with the guest QXL driver:

Run migration source:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-src.sock

$EMULATOR -enable-kvm \
       -machine q35 \
       -cpu host -smp 2 -m 2G \
       -object
memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/
ram0,share=on\
       -machine memory-backend=ram0 \
       -machine aux-ram-share=on \
       -drive file=$ROOTFS,media=disk,if=virtio \
       -qmp unix:$QMPSOCK,server=on,wait=off \
       -nographic \
       -device qxl-vga
Run migration target:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-dst.sock
$EMULATOR -enable-kvm \
       -machine q35 \
       -cpu host -smp 2 -m 2G \
       -object
memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/
ram0,share=on\
       -machine memory-backend=ram0 \
       -machine aux-ram-share=on \
       -drive file=$ROOTFS,media=disk,if=virtio \
       -qmp unix:$QMPSOCK,server=on,wait=off \
       -nographic \
       -device qxl-vga \
       -incoming tcp:0:44444 \
       -incoming '{"channel-type": "cpr", "addr": { "transport":
"socket", "type": "unix", "path": "/var/run/alma8cpr-dst.sock"}}'
Launch the migration:
QMPSHELL=/root/src/qemu/master/scripts/qmp/qmp-shell
QMPSOCK=/var/run/alma8qmp-src.sock

$QMPSHELL -p $QMPSOCK <<EOF
       migrate-set-parameters mode=cpr-transfer
       migrate channels=[{"channel-type":"main","addr":
{"transport":"socket","type":"inet","host":"0","port":"44444"}},
{"channel-type":"cpr","addr":
{"transport":"socket","type":"unix","path":"/var/run/alma8cpr-
dst.sock"}}]
EOF
Then, after a while, QXL guest driver on target crashes spewing
the
following messages:
[   73.962002] [TTM] Buffer eviction failed
[   73.962072] qxl 0000:00:02.0: object_init failed for (3149824,
0x00000001)
[   73.962081] [drm:qxl_alloc_bo_reserved [qxl]] *ERROR*
failed to
allocate VRAM BO
That seems to be a known kernel QXL driver bug:
https://lore.kernel.org/all/20220907094423.93581-1-
min_halo@163.com/T/
https://lore.kernel.org/lkml/ZTgydqRlK6WX_b29@eldamar.lan/
(the latter discussion contains that reproduce script which
speeds up
the crash in the guest):
#!/bin/bash

chvt 3

for j in $(seq 80); do
           echo "$(date) starting round $j"
           if [ "$(journalctl --boot | grep "failed to
allocate VRAM
BO")" != "" ]; then
                   echo "bug was reproduced after $j tries"
                   exit 1
           fi
           for i in $(seq 100); do
                   dmesg > /dev/tty3
           done
done

echo "bug could not be reproduced"
exit 0
The bug itself seems to remain unfixed, as I was able to reproduce
that
with Fedora 41 guest, as well as AlmaLinux 8 guest. However our
cpr-transfer code also seems to be buggy as it triggers the
crash -
without the cpr-transfer migration the above reproduce doesn't
lead to
crash on the source VM.
I suspect that, as cpr-transfer doesn't migrate the guest
memory, but
rather passes it through the memory backend object, our code might
somehow corrupt the VRAM.  However, I wasn't able to trace the
corruption so far.
Could somebody help the investigation and take a look into
this?  Any
suggestions would be appreciated.  Thanks!
Possibly some memory region created by qxl is not being preserved.
Try adding these traces to see what is preserved:

-trace enable='*cpr*'
-trace enable='*ram_alloc*'
Also try adding this patch to see if it flags any ram blocks as not
compatible with cpr.  A message is printed at migration start time.
https://lore.kernel.org/qemu-devel/1740667681-257312-1-git-send-
email-
steven.sistare@oracle.com/

- Steve
With the traces enabled + the "migration: ram block cpr blockers"
patch
applied:

Source:
cpr_find_fd pc.bios, id 0 returns -1
cpr_save_fd pc.bios, id 0, fd 22
qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 22 host
0x7fec18e00000
cpr_find_fd pc.rom, id 0 returns -1
cpr_save_fd pc.rom, id 0, fd 23
qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 23 host
0x7fec18c00000
cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns -1
cpr_save_fd 0000:00:01.0/e1000e.rom, id 0, fd 24
qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size
262144 fd 24 host 0x7fec18a00000
cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns -1
cpr_save_fd 0000:00:02.0/vga.vram, id 0, fd 25
qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size
67108864 fd 25 host 0x7feb77e00000
cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.vrom, id 0, fd 27
qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192
fd 27 host 0x7fec18800000
cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.vram, id 0, fd 28
qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size
67108864 fd 28 host 0x7feb73c00000
cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.rom, id 0, fd 34
qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536
fd 34 host 0x7fec18600000
cpr_find_fd /rom@etc/acpi/tables, id 0 returns -1
cpr_save_fd /rom@etc/acpi/tables, id 0, fd 35
qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size
2097152 fd 35 host 0x7fec18200000
cpr_find_fd /rom@etc/table-loader, id 0 returns -1
cpr_save_fd /rom@etc/table-loader, id 0, fd 36
qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536
fd 36 host 0x7feb8b600000
cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns -1
cpr_save_fd /rom@etc/acpi/rsdp, id 0, fd 37
qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd
37 host 0x7feb8b400000

cpr_state_save cpr-transfer mode
cpr_transfer_output /var/run/alma8cpr-dst.sock
Target:
cpr_transfer_input /var/run/alma8cpr-dst.sock
cpr_state_load cpr-transfer mode
cpr_find_fd pc.bios, id 0 returns 20
qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 20 host
0x7fcdc9800000
cpr_find_fd pc.rom, id 0 returns 19
qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 19 host
0x7fcdc9600000
cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns 18
qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size
262144 fd 18 host 0x7fcdc9400000
cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns 17
qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size
67108864 fd 17 host 0x7fcd27e00000
cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns 16
qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192
fd 16 host 0x7fcdc9200000
cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns 15
qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size
67108864 fd 15 host 0x7fcd23c00000
cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns 14
qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536
fd 14 host 0x7fcdc8800000
cpr_find_fd /rom@etc/acpi/tables, id 0 returns 13
qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size
2097152 fd 13 host 0x7fcdc8400000
cpr_find_fd /rom@etc/table-loader, id 0 returns 11
qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536
fd 11 host 0x7fcdc8200000
cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns 10
qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd
10 host 0x7fcd3be00000
Looks like both vga.vram and qxl.vram are being preserved (with
the same
addresses), and no incompatible ram blocks are found during
migration.
Sorry, addressed are not the same, of course.  However
corresponding ram
blocks do seem to be preserved and initialized.
So far, I have not reproduced the guest driver failure.

However, I have isolated places where new QEMU improperly writes to
the qxl memory regions prior to starting the guest, by mmap'ing them
readonly after cpr:

   qemu_ram_alloc_internal()
     if (reused && (strstr(name, "qxl") || strstr("name", "vga")))
         ram_flags |= RAM_READONLY;
     new_block = qemu_ram_alloc_from_fd(...)

I have attached a draft fix; try it and let me know.
My console window looks fine before and after cpr, using
-vnc $hostip:0 -vga qxl

- Steve
Regarding the reproduce: when I launch the buggy version with the same
options as you, i.e. "-vnc 0.0.0.0:$port -vga qxl", and do cpr-transfer,
my VNC client silently hangs on the target after a while.  Could it
happen on your stand as well?
cpr does not preserve the vnc connection and session.  To test, I specify
port 0 for the source VM and port 1 for the dest.  When the src vnc goes
dormant the dest vnc becomes active.
Could you try launching VM with
"-nographic -device qxl-vga"?  That way VM's serial console is given you
directly in the shell, so when qxl driver crashes you're still able to
inspect the kernel messages.
I have been running like that, but have not reproduced the qxl driver
crash,
and I suspect my guest image+kernel is too old.  However, once I
realized the
issue was post-cpr modification of qxl memory, I switched my attention
to the
fix.
As for your patch, I can report that it doesn't resolve the issue as it
is.  But I was able to track down another possible memory corruption
using your approach with readonly mmap'ing:
Program terminated with signal SIGSEGV, Segmentation fault.
#0  init_qxl_ram (d=0x5638996e0e70) at ../hw/display/qxl.c:412
412         d->ram->magic       = cpu_to_le32(QXL_RAM_MAGIC);
[Current thread is 1 (Thread 0x7f1a4f83b480 (LWP 229798))]
(gdb) bt
#0  init_qxl_ram (d=0x5638996e0e70) at ../hw/display/qxl.c:412
#1  0x0000563896e7f467 in qxl_realize_common (qxl=0x5638996e0e70,
errp=0x7ffd3c2b8170) at ../hw/display/qxl.c:2142
#2  0x0000563896e7fda1 in qxl_realize_primary (dev=0x5638996e0e70,
errp=0x7ffd3c2b81d0) at ../hw/display/qxl.c:2257
#3  0x0000563896c7e8f2 in pci_qdev_realize (qdev=0x5638996e0e70,
errp=0x7ffd3c2b8250) at ../hw/pci/pci.c:2174
#4  0x00005638970eb54b in device_set_realized (obj=0x5638996e0e70,
value=true, errp=0x7ffd3c2b84e0) at ../hw/core/qdev.c:494
#5  0x00005638970f5e14 in property_set_bool (obj=0x5638996e0e70,
v=0x5638996f3770, name=0x56389759b141 "realized",
opaque=0x5638987893d0, errp=0x7ffd3c2b84e0)
     at ../qom/object.c:2374
#6  0x00005638970f39f8 in object_property_set (obj=0x5638996e0e70,
name=0x56389759b141 "realized", v=0x5638996f3770, errp=0x7ffd3c2b84e0)
     at ../qom/object.c:1449
#7  0x00005638970f8586 in object_property_set_qobject
(obj=0x5638996e0e70, name=0x56389759b141 "realized",
value=0x5638996df900, errp=0x7ffd3c2b84e0)
     at ../qom/qom-qobject.c:28
#8  0x00005638970f3d8d in object_property_set_bool
(obj=0x5638996e0e70, name=0x56389759b141 "realized", value=true,
errp=0x7ffd3c2b84e0)
     at ../qom/object.c:1519
#9  0x00005638970eacb0 in qdev_realize (dev=0x5638996e0e70,
bus=0x563898cf3c20, errp=0x7ffd3c2b84e0) at ../hw/core/qdev.c:276
#10 0x0000563896dba675 in qdev_device_add_from_qdict
(opts=0x5638996dfe50, from_json=false, errp=0x7ffd3c2b84e0) at
../system/qdev-monitor.c:714
#11 0x0000563896dba721 in qdev_device_add (opts=0x563898786150,
errp=0x56389855dc40 <error_fatal>) at ../system/qdev-monitor.c:733
#12 0x0000563896dc48f1 in device_init_func (opaque=0x0,
opts=0x563898786150, errp=0x56389855dc40 <error_fatal>) at
../system/vl.c:1207
#13 0x000056389737a6cc in qemu_opts_foreach
     (list=0x563898427b60 <qemu_device_opts>, func=0x563896dc48ca
<device_init_func>, opaque=0x0, errp=0x56389855dc40 <error_fatal>)
     at ../util/qemu-option.c:1135
#14 0x0000563896dc89b5 in qemu_create_cli_devices () at
../system/vl.c:2745
#15 0x0000563896dc8c00 in qmp_x_exit_preconfig (errp=0x56389855dc40
<error_fatal>) at ../system/vl.c:2806
#16 0x0000563896dcb5de in qemu_init (argc=33, argv=0x7ffd3c2b8948)
at ../system/vl.c:3838
#17 0x0000563897297323 in main (argc=33, argv=0x7ffd3c2b8948) at
../system/main.c:72
So the attached adjusted version of your patch does seem to help.  At
least I can't reproduce the crash on my stand.
Thanks for the stack trace; the calls to SPICE_RING_INIT in
init_qxl_ram are
definitely harmful.  Try V2 of the patch, attached, which skips the lines
of init_qxl_ram that modify guest memory.
I'm wondering, could it be useful to explicitly mark all the reused
memory regions readonly upon cpr-transfer, and then make them writable
back again after the migration is done?  That way we will be segfaulting
early on instead of debugging tricky memory corruptions.
It's a useful debugging technique, but changing protection on a large
memory region
can be too expensive for production due to TLB shootdowns.
Good point. Though we could move this code under non-default option to
avoid re-writing.

Den

On 3/5/25 11:19 PM, Steven Sistare wrote:
>
On 3/5/2025 11:50 AM, Andrey Drobyshev wrote:
>
> On 3/4/25 9:05 PM, Steven Sistare wrote:
>
>> On 2/28/2025 1:37 PM, Andrey Drobyshev wrote:
>
>>> On 2/28/25 8:35 PM, Andrey Drobyshev wrote:
>
>>>> On 2/28/25 8:20 PM, Steven Sistare wrote:
>
>>>>> On 2/28/2025 1:13 PM, Steven Sistare wrote:
>
>>>>>> On 2/28/2025 12:39 PM, Andrey Drobyshev wrote:
>
>>>>>>> Hi all,
>
>>>>>>>
>
>>>>>>> We've been experimenting with cpr-transfer migration mode recently
>
>>>>>>> and
>
>>>>>>> have discovered the following issue with the guest QXL driver:
>
>>>>>>>
>
>>>>>>> Run migration source:
>
>>>>>>>> EMULATOR=/path/to/emulator
>
>>>>>>>> ROOTFS=/path/to/image
>
>>>>>>>> QMPSOCK=/var/run/alma8qmp-src.sock
>
>>>>>>>>
>
>>>>>>>> $EMULATOR -enable-kvm \
>
>>>>>>>>        -machine q35 \
>
>>>>>>>>        -cpu host -smp 2 -m 2G \
>
>>>>>>>>        -object memory-backend-file,id=ram0,size=2G,mem-path=/
>
>>>>>>>> dev/shm/
>
>>>>>>>> ram0,share=on\
>
>>>>>>>>        -machine memory-backend=ram0 \
>
>>>>>>>>        -machine aux-ram-share=on \
>
>>>>>>>>        -drive file=$ROOTFS,media=disk,if=virtio \
>
>>>>>>>>        -qmp unix:$QMPSOCK,server=on,wait=off \
>
>>>>>>>>        -nographic \
>
>>>>>>>>        -device qxl-vga
>
>>>>>>>
>
>>>>>>> Run migration target:
>
>>>>>>>> EMULATOR=/path/to/emulator
>
>>>>>>>> ROOTFS=/path/to/image
>
>>>>>>>> QMPSOCK=/var/run/alma8qmp-dst.sock
>
>>>>>>>> $EMULATOR -enable-kvm \
>
>>>>>>>>        -machine q35 \
>
>>>>>>>>        -cpu host -smp 2 -m 2G \
>
>>>>>>>>        -object memory-backend-file,id=ram0,size=2G,mem-path=/
>
>>>>>>>> dev/shm/
>
>>>>>>>> ram0,share=on\
>
>>>>>>>>        -machine memory-backend=ram0 \
>
>>>>>>>>        -machine aux-ram-share=on \
>
>>>>>>>>        -drive file=$ROOTFS,media=disk,if=virtio \
>
>>>>>>>>        -qmp unix:$QMPSOCK,server=on,wait=off \
>
>>>>>>>>        -nographic \
>
>>>>>>>>        -device qxl-vga \
>
>>>>>>>>        -incoming tcp:0:44444 \
>
>>>>>>>>        -incoming '{"channel-type": "cpr", "addr": { "transport":
>
>>>>>>>> "socket", "type": "unix", "path": "/var/run/alma8cpr-dst.sock"}}'
>
>>>>>>>
>
>>>>>>>
>
>>>>>>> Launch the migration:
>
>>>>>>>> QMPSHELL=/root/src/qemu/master/scripts/qmp/qmp-shell
>
>>>>>>>> QMPSOCK=/var/run/alma8qmp-src.sock
>
>>>>>>>>
>
>>>>>>>> $QMPSHELL -p $QMPSOCK <<EOF
>
>>>>>>>>        migrate-set-parameters mode=cpr-transfer
>
>>>>>>>>        migrate channels=[{"channel-type":"main","addr":
>
>>>>>>>> {"transport":"socket","type":"inet","host":"0","port":"44444"}},
>
>>>>>>>> {"channel-type":"cpr","addr":
>
>>>>>>>> {"transport":"socket","type":"unix","path":"/var/run/alma8cpr-
>
>>>>>>>> dst.sock"}}]
>
>>>>>>>> EOF
>
>>>>>>>
>
>>>>>>> Then, after a while, QXL guest driver on target crashes spewing the
>
>>>>>>> following messages:
>
>>>>>>>> [   73.962002] [TTM] Buffer eviction failed
>
>>>>>>>> [   73.962072] qxl 0000:00:02.0: object_init failed for (3149824,
>
>>>>>>>> 0x00000001)
>
>>>>>>>> [   73.962081] [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to
>
>>>>>>>> allocate VRAM BO
>
>>>>>>>
>
>>>>>>> That seems to be a known kernel QXL driver bug:
>
>>>>>>>
>
>>>>>>>
https://lore.kernel.org/all/20220907094423.93581-1-
>
>>>>>>> min_halo@163.com/T/
>
>>>>>>>
https://lore.kernel.org/lkml/ZTgydqRlK6WX_b29@eldamar.lan/
>
>>>>>>>
>
>>>>>>> (the latter discussion contains that reproduce script which
>
>>>>>>> speeds up
>
>>>>>>> the crash in the guest):
>
>>>>>>>> #!/bin/bash
>
>>>>>>>>
>
>>>>>>>> chvt 3
>
>>>>>>>>
>
>>>>>>>> for j in $(seq 80); do
>
>>>>>>>>            echo "$(date) starting round $j"
>
>>>>>>>>            if [ "$(journalctl --boot | grep "failed to allocate
>
>>>>>>>> VRAM
>
>>>>>>>> BO")" != "" ]; then
>
>>>>>>>>                    echo "bug was reproduced after $j tries"
>
>>>>>>>>                    exit 1
>
>>>>>>>>            fi
>
>>>>>>>>            for i in $(seq 100); do
>
>>>>>>>>                    dmesg > /dev/tty3
>
>>>>>>>>            done
>
>>>>>>>> done
>
>>>>>>>>
>
>>>>>>>> echo "bug could not be reproduced"
>
>>>>>>>> exit 0
>
>>>>>>>
>
>>>>>>> The bug itself seems to remain unfixed, as I was able to reproduce
>
>>>>>>> that
>
>>>>>>> with Fedora 41 guest, as well as AlmaLinux 8 guest. However our
>
>>>>>>> cpr-transfer code also seems to be buggy as it triggers the crash -
>
>>>>>>> without the cpr-transfer migration the above reproduce doesn't
>
>>>>>>> lead to
>
>>>>>>> crash on the source VM.
>
>>>>>>>
>
>>>>>>> I suspect that, as cpr-transfer doesn't migrate the guest
>
>>>>>>> memory, but
>
>>>>>>> rather passes it through the memory backend object, our code might
>
>>>>>>> somehow corrupt the VRAM.  However, I wasn't able to trace the
>
>>>>>>> corruption so far.
>
>>>>>>>
>
>>>>>>> Could somebody help the investigation and take a look into
>
>>>>>>> this?  Any
>
>>>>>>> suggestions would be appreciated.  Thanks!
>
>>>>>>
>
>>>>>> Possibly some memory region created by qxl is not being preserved.
>
>>>>>> Try adding these traces to see what is preserved:
>
>>>>>>
>
>>>>>> -trace enable='*cpr*'
>
>>>>>> -trace enable='*ram_alloc*'
>
>>>>>
>
>>>>> Also try adding this patch to see if it flags any ram blocks as not
>
>>>>> compatible with cpr.  A message is printed at migration start time.
>
>>>>>   Â
https://lore.kernel.org/qemu-devel/1740667681-257312-1-git-send-
>
>>>>> email-
>
>>>>> steven.sistare@oracle.com/
>
>>>>>
>
>>>>> - Steve
>
>>>>>
>
>>>>
>
>>>> With the traces enabled + the "migration: ram block cpr blockers"
>
>>>> patch
>
>>>> applied:
>
>>>>
>
>>>> Source:
>
>>>>> cpr_find_fd pc.bios, id 0 returns -1
>
>>>>> cpr_save_fd pc.bios, id 0, fd 22
>
>>>>> qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 22 host
>
>>>>> 0x7fec18e00000
>
>>>>> cpr_find_fd pc.rom, id 0 returns -1
>
>>>>> cpr_save_fd pc.rom, id 0, fd 23
>
>>>>> qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 23 host
>
>>>>> 0x7fec18c00000
>
>>>>> cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns -1
>
>>>>> cpr_save_fd 0000:00:01.0/e1000e.rom, id 0, fd 24
>
>>>>> qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size
>
>>>>> 262144 fd 24 host 0x7fec18a00000
>
>>>>> cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns -1
>
>>>>> cpr_save_fd 0000:00:02.0/vga.vram, id 0, fd 25
>
>>>>> qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size
>
>>>>> 67108864 fd 25 host 0x7feb77e00000
>
>>>>> cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns -1
>
>>>>> cpr_save_fd 0000:00:02.0/qxl.vrom, id 0, fd 27
>
>>>>> qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192
>
>>>>> fd 27 host 0x7fec18800000
>
>>>>> cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns -1
>
>>>>> cpr_save_fd 0000:00:02.0/qxl.vram, id 0, fd 28
>
>>>>> qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size
>
>>>>> 67108864 fd 28 host 0x7feb73c00000
>
>>>>> cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns -1
>
>>>>> cpr_save_fd 0000:00:02.0/qxl.rom, id 0, fd 34
>
>>>>> qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536
>
>>>>> fd 34 host 0x7fec18600000
>
>>>>> cpr_find_fd /rom@etc/acpi/tables, id 0 returns -1
>
>>>>> cpr_save_fd /rom@etc/acpi/tables, id 0, fd 35
>
>>>>> qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size
>
>>>>> 2097152 fd 35 host 0x7fec18200000
>
>>>>> cpr_find_fd /rom@etc/table-loader, id 0 returns -1
>
>>>>> cpr_save_fd /rom@etc/table-loader, id 0, fd 36
>
>>>>> qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536
>
>>>>> fd 36 host 0x7feb8b600000
>
>>>>> cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns -1
>
>>>>> cpr_save_fd /rom@etc/acpi/rsdp, id 0, fd 37
>
>>>>> qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd
>
>>>>> 37 host 0x7feb8b400000
>
>>>>>
>
>>>>> cpr_state_save cpr-transfer mode
>
>>>>> cpr_transfer_output /var/run/alma8cpr-dst.sock
>
>>>>
>
>>>> Target:
>
>>>>> cpr_transfer_input /var/run/alma8cpr-dst.sock
>
>>>>> cpr_state_load cpr-transfer mode
>
>>>>> cpr_find_fd pc.bios, id 0 returns 20
>
>>>>> qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 20 host
>
>>>>> 0x7fcdc9800000
>
>>>>> cpr_find_fd pc.rom, id 0 returns 19
>
>>>>> qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 19 host
>
>>>>> 0x7fcdc9600000
>
>>>>> cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns 18
>
>>>>> qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size
>
>>>>> 262144 fd 18 host 0x7fcdc9400000
>
>>>>> cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns 17
>
>>>>> qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size
>
>>>>> 67108864 fd 17 host 0x7fcd27e00000
>
>>>>> cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns 16
>
>>>>> qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192
>
>>>>> fd 16 host 0x7fcdc9200000
>
>>>>> cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns 15
>
>>>>> qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size
>
>>>>> 67108864 fd 15 host 0x7fcd23c00000
>
>>>>> cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns 14
>
>>>>> qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536
>
>>>>> fd 14 host 0x7fcdc8800000
>
>>>>> cpr_find_fd /rom@etc/acpi/tables, id 0 returns 13
>
>>>>> qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size
>
>>>>> 2097152 fd 13 host 0x7fcdc8400000
>
>>>>> cpr_find_fd /rom@etc/table-loader, id 0 returns 11
>
>>>>> qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536
>
>>>>> fd 11 host 0x7fcdc8200000
>
>>>>> cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns 10
>
>>>>> qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd
>
>>>>> 10 host 0x7fcd3be00000
>
>>>>
>
>>>> Looks like both vga.vram and qxl.vram are being preserved (with the
>
>>>> same
>
>>>> addresses), and no incompatible ram blocks are found during migration.
>
>>>
>
>>> Sorry, addressed are not the same, of course.  However corresponding
>
>>> ram
>
>>> blocks do seem to be preserved and initialized.
>
>>
>
>> So far, I have not reproduced the guest driver failure.
>
>>
>
>> However, I have isolated places where new QEMU improperly writes to
>
>> the qxl memory regions prior to starting the guest, by mmap'ing them
>
>> readonly after cpr:
>
>>
>
>>    qemu_ram_alloc_internal()
>
>>      if (reused && (strstr(name, "qxl") || strstr("name", "vga")))
>
>>          ram_flags |= RAM_READONLY;
>
>>      new_block = qemu_ram_alloc_from_fd(...)
>
>>
>
>> I have attached a draft fix; try it and let me know.
>
>> My console window looks fine before and after cpr, using
>
>> -vnc $hostip:0 -vga qxl
>
>>
>
>> - Steve
>
>
>
> Regarding the reproduce: when I launch the buggy version with the same
>
> options as you, i.e. "-vnc 0.0.0.0:$port -vga qxl", and do cpr-transfer,
>
> my VNC client silently hangs on the target after a while.  Could it
>
> happen on your stand as well?Â
>
>
cpr does not preserve the vnc connection and session.  To test, I specify
>
port 0 for the source VM and port 1 for the dest.  When the src vnc goes
>
dormant the dest vnc becomes active.
>
Sure, I meant that VNC on the dest (on the port 1) works for a while
after the migration and then hangs, apparently after the guest QXL crash.

>
> Could you try launching VM with
>
> "-nographic -device qxl-vga"?  That way VM's serial console is given you
>
> directly in the shell, so when qxl driver crashes you're still able to
>
> inspect the kernel messages.
>
>
I have been running like that, but have not reproduced the qxl driver
>
crash,
>
and I suspect my guest image+kernel is too old.
Yes, that's probably the case.  But the crash occurs on my Fedora 41
guest with the 6.11.5-300.fc41.x86_64 kernel, so newer kernels seem to
be buggy.


>
However, once I realized the
>
issue was post-cpr modification of qxl memory, I switched my attention
>
to the
>
fix.
>
>
> As for your patch, I can report that it doesn't resolve the issue as it
>
> is.  But I was able to track down another possible memory corruption
>
> using your approach with readonly mmap'ing:
>
>
>
>> Program terminated with signal SIGSEGV, Segmentation fault.
>
>> #0  init_qxl_ram (d=0x5638996e0e70) at ../hw/display/qxl.c:412
>
>> 412         d->ram->magic       = cpu_to_le32(QXL_RAM_MAGIC);
>
>> [Current thread is 1 (Thread 0x7f1a4f83b480 (LWP 229798))]
>
>> (gdb) bt
>
>> #0  init_qxl_ram (d=0x5638996e0e70) at ../hw/display/qxl.c:412
>
>> #1  0x0000563896e7f467 in qxl_realize_common (qxl=0x5638996e0e70,
>
>> errp=0x7ffd3c2b8170) at ../hw/display/qxl.c:2142
>
>> #2  0x0000563896e7fda1 in qxl_realize_primary (dev=0x5638996e0e70,
>
>> errp=0x7ffd3c2b81d0) at ../hw/display/qxl.c:2257
>
>> #3  0x0000563896c7e8f2 in pci_qdev_realize (qdev=0x5638996e0e70,
>
>> errp=0x7ffd3c2b8250) at ../hw/pci/pci.c:2174
>
>> #4  0x00005638970eb54b in device_set_realized (obj=0x5638996e0e70,
>
>> value=true, errp=0x7ffd3c2b84e0) at ../hw/core/qdev.c:494
>
>> #5  0x00005638970f5e14 in property_set_bool (obj=0x5638996e0e70,
>
>> v=0x5638996f3770, name=0x56389759b141 "realized",
>
>> opaque=0x5638987893d0, errp=0x7ffd3c2b84e0)
>
>>      at ../qom/object.c:2374
>
>> #6  0x00005638970f39f8 in object_property_set (obj=0x5638996e0e70,
>
>> name=0x56389759b141 "realized", v=0x5638996f3770, errp=0x7ffd3c2b84e0)
>
>>      at ../qom/object.c:1449
>
>> #7  0x00005638970f8586 in object_property_set_qobject
>
>> (obj=0x5638996e0e70, name=0x56389759b141 "realized",
>
>> value=0x5638996df900, errp=0x7ffd3c2b84e0)
>
>>      at ../qom/qom-qobject.c:28
>
>> #8  0x00005638970f3d8d in object_property_set_bool
>
>> (obj=0x5638996e0e70, name=0x56389759b141 "realized", value=true,
>
>> errp=0x7ffd3c2b84e0)
>
>>      at ../qom/object.c:1519
>
>> #9  0x00005638970eacb0 in qdev_realize (dev=0x5638996e0e70,
>
>> bus=0x563898cf3c20, errp=0x7ffd3c2b84e0) at ../hw/core/qdev.c:276
>
>> #10 0x0000563896dba675 in qdev_device_add_from_qdict
>
>> (opts=0x5638996dfe50, from_json=false, errp=0x7ffd3c2b84e0) at ../
>
>> system/qdev-monitor.c:714
>
>> #11 0x0000563896dba721 in qdev_device_add (opts=0x563898786150,
>
>> errp=0x56389855dc40 <error_fatal>) at ../system/qdev-monitor.c:733
>
>> #12 0x0000563896dc48f1 in device_init_func (opaque=0x0,
>
>> opts=0x563898786150, errp=0x56389855dc40 <error_fatal>) at ../system/
>
>> vl.c:1207
>
>> #13 0x000056389737a6cc in qemu_opts_foreach
>
>>      (list=0x563898427b60 <qemu_device_opts>, func=0x563896dc48ca
>
>> <device_init_func>, opaque=0x0, errp=0x56389855dc40 <error_fatal>)
>
>>      at ../util/qemu-option.c:1135
>
>> #14 0x0000563896dc89b5 in qemu_create_cli_devices () at ../system/
>
>> vl.c:2745
>
>> #15 0x0000563896dc8c00 in qmp_x_exit_preconfig (errp=0x56389855dc40
>
>> <error_fatal>) at ../system/vl.c:2806
>
>> #16 0x0000563896dcb5de in qemu_init (argc=33, argv=0x7ffd3c2b8948)
>
>> at ../system/vl.c:3838
>
>> #17 0x0000563897297323 in main (argc=33, argv=0x7ffd3c2b8948) at ../
>
>> system/main.c:72
>
>
>
> So the attached adjusted version of your patch does seem to help.  At
>
> least I can't reproduce the crash on my stand.
>
>
Thanks for the stack trace; the calls to SPICE_RING_INIT in init_qxl_ram
>
are
>
definitely harmful.  Try V2 of the patch, attached, which skips the lines
>
of init_qxl_ram that modify guest memory.
>
Thanks, your v2 patch does seem to prevent the crash.  Would you re-send
it to the list as a proper fix?

>
> I'm wondering, could it be useful to explicitly mark all the reused
>
> memory regions readonly upon cpr-transfer, and then make them writable
>
> back again after the migration is done?  That way we will be segfaulting
>
> early on instead of debugging tricky memory corruptions.
>
>
It's a useful debugging technique, but changing protection on a large
>
memory region
>
can be too expensive for production due to TLB shootdowns.
>
>
Also, there are cases where writes are performed but the value is
>
guaranteed to
>
be the same:
>
  qxl_post_load()
>
    qxl_set_mode()
>
      d->rom->mode = cpu_to_le32(modenr);
>
The value is the same because mode and shadow_rom.mode were passed in
>
vmstate
>
from old qemu.
>
There're also cases where devices' ROM might be re-initialized.  E.g.
this segfault occures upon further exploration of RO mapped RAM blocks:

>
Program terminated with signal SIGSEGV, Segmentation fault.
>
#0  __memmove_avx_unaligned_erms () at
>
../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:664
>
664             rep     movsb
>
[Current thread is 1 (Thread 0x7f6e7d08b480 (LWP 310379))]
>
(gdb) bt
>
#0  __memmove_avx_unaligned_erms () at
>
../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:664
>
#1  0x000055aa1d030ecd in rom_set_mr (rom=0x55aa200ba380,
>
owner=0x55aa2019ac10, name=0x7fffb8272bc0 "/rom@etc/acpi/tables", ro=true)
>
at ../hw/core/loader.c:1032
>
#2  0x000055aa1d031577 in rom_add_blob
>
(name=0x55aa1da51f13 "etc/acpi/tables", blob=0x55aa208a1070, len=131072,
>
max_len=2097152, addr=18446744073709551615, fw_file_name=0x55aa1da51f13
>
"etc/acpi/tables", fw_callback=0x55aa1d441f59 <acpi_build_update>,
>
callback_opaque=0x55aa20ff0010, as=0x0, read_only=true) at
>
../hw/core/loader.c:1147
>
#3  0x000055aa1cfd788d in acpi_add_rom_blob
>
(update=0x55aa1d441f59 <acpi_build_update>, opaque=0x55aa20ff0010,
>
blob=0x55aa1fc9aa00, name=0x55aa1da51f13 "etc/acpi/tables") at
>
../hw/acpi/utils.c:46
>
#4  0x000055aa1d44213f in acpi_setup () at ../hw/i386/acpi-build.c:2720
>
#5  0x000055aa1d434199 in pc_machine_done (notifier=0x55aa1ff15050, data=0x0)
>
at ../hw/i386/pc.c:638
>
#6  0x000055aa1d876845 in notifier_list_notify (list=0x55aa1ea25c10
>
<machine_init_done_notifiers>, data=0x0) at ../util/notify.c:39
>
#7  0x000055aa1d039ee5 in qdev_machine_creation_done () at
>
../hw/core/machine.c:1749
>
#8  0x000055aa1d2c7b3e in qemu_machine_creation_done (errp=0x55aa1ea5cc40
>
<error_fatal>) at ../system/vl.c:2779
>
#9  0x000055aa1d2c7c7d in qmp_x_exit_preconfig (errp=0x55aa1ea5cc40
>
<error_fatal>) at ../system/vl.c:2807
>
#10 0x000055aa1d2ca64f in qemu_init (argc=35, argv=0x7fffb82730e8) at
>
../system/vl.c:3838
>
#11 0x000055aa1d79638c in main (argc=35, argv=0x7fffb82730e8) at
>
../system/main.c:72
I'm not sure whether ACPI tables ROM in particular is rewritten with the
same content, but there might be cases where ROM can be read from file
system upon initialization.  That is undesirable as guest kernel
certainly won't be too happy about sudden change of the device's ROM
content.

So the issue we're dealing with here is any unwanted memory related
device initialization upon cpr.

For now the only thing that comes to my mind is to make a test where we
put as many devices as we can into a VM, make ram blocks RO upon cpr
(and remap them as RW later after migration is done, if needed), and
catch any unwanted memory violations.  As Den suggested, we might
consider adding that behaviour as a separate non-default option (or
"migrate" command flag specific to cpr-transfer), which would only be
used in the testing.

Andrey

On 3/6/25 16:16, Andrey Drobyshev wrote:
On 3/5/25 11:19 PM, Steven Sistare wrote:
On 3/5/2025 11:50 AM, Andrey Drobyshev wrote:
On 3/4/25 9:05 PM, Steven Sistare wrote:
On 2/28/2025 1:37 PM, Andrey Drobyshev wrote:
On 2/28/25 8:35 PM, Andrey Drobyshev wrote:
On 2/28/25 8:20 PM, Steven Sistare wrote:
On 2/28/2025 1:13 PM, Steven Sistare wrote:
On 2/28/2025 12:39 PM, Andrey Drobyshev wrote:
Hi all,

We've been experimenting with cpr-transfer migration mode recently
and
have discovered the following issue with the guest QXL driver:

Run migration source:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-src.sock

$EMULATOR -enable-kvm \
        -machine q35 \
        -cpu host -smp 2 -m 2G \
        -object memory-backend-file,id=ram0,size=2G,mem-path=/
dev/shm/
ram0,share=on\
        -machine memory-backend=ram0 \
        -machine aux-ram-share=on \
        -drive file=$ROOTFS,media=disk,if=virtio \
        -qmp unix:$QMPSOCK,server=on,wait=off \
        -nographic \
        -device qxl-vga
Run migration target:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-dst.sock
$EMULATOR -enable-kvm \
        -machine q35 \
        -cpu host -smp 2 -m 2G \
        -object memory-backend-file,id=ram0,size=2G,mem-path=/
dev/shm/
ram0,share=on\
        -machine memory-backend=ram0 \
        -machine aux-ram-share=on \
        -drive file=$ROOTFS,media=disk,if=virtio \
        -qmp unix:$QMPSOCK,server=on,wait=off \
        -nographic \
        -device qxl-vga \
        -incoming tcp:0:44444 \
        -incoming '{"channel-type": "cpr", "addr": { "transport":
"socket", "type": "unix", "path": "/var/run/alma8cpr-dst.sock"}}'
Launch the migration:
QMPSHELL=/root/src/qemu/master/scripts/qmp/qmp-shell
QMPSOCK=/var/run/alma8qmp-src.sock

$QMPSHELL -p $QMPSOCK <<EOF
        migrate-set-parameters mode=cpr-transfer
        migrate channels=[{"channel-type":"main","addr":
{"transport":"socket","type":"inet","host":"0","port":"44444"}},
{"channel-type":"cpr","addr":
{"transport":"socket","type":"unix","path":"/var/run/alma8cpr-
dst.sock"}}]
EOF
Then, after a while, QXL guest driver on target crashes spewing the
following messages:
[   73.962002] [TTM] Buffer eviction failed
[   73.962072] qxl 0000:00:02.0: object_init failed for (3149824,
0x00000001)
[   73.962081] [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to
allocate VRAM BO
That seems to be a known kernel QXL driver bug:
https://lore.kernel.org/all/20220907094423.93581-1-
min_halo@163.com/T/
https://lore.kernel.org/lkml/ZTgydqRlK6WX_b29@eldamar.lan/
(the latter discussion contains that reproduce script which
speeds up
the crash in the guest):
#!/bin/bash

chvt 3

for j in $(seq 80); do
            echo "$(date) starting round $j"
            if [ "$(journalctl --boot | grep "failed to allocate
VRAM
BO")" != "" ]; then
                    echo "bug was reproduced after $j tries"
                    exit 1
            fi
            for i in $(seq 100); do
                    dmesg > /dev/tty3
            done
done

echo "bug could not be reproduced"
exit 0
The bug itself seems to remain unfixed, as I was able to reproduce
that
with Fedora 41 guest, as well as AlmaLinux 8 guest. However our
cpr-transfer code also seems to be buggy as it triggers the crash -
without the cpr-transfer migration the above reproduce doesn't
lead to
crash on the source VM.

I suspect that, as cpr-transfer doesn't migrate the guest
memory, but
rather passes it through the memory backend object, our code might
somehow corrupt the VRAM.  However, I wasn't able to trace the
corruption so far.

Could somebody help the investigation and take a look into
this?  Any
suggestions would be appreciated.  Thanks!
Possibly some memory region created by qxl is not being preserved.
Try adding these traces to see what is preserved:

-trace enable='*cpr*'
-trace enable='*ram_alloc*'
Also try adding this patch to see if it flags any ram blocks as not
compatible with cpr.  A message is printed at migration start time.
   Â
https://lore.kernel.org/qemu-devel/1740667681-257312-1-git-send-
email-
steven.sistare@oracle.com/

- Steve
With the traces enabled + the "migration: ram block cpr blockers"
patch
applied:

Source:
cpr_find_fd pc.bios, id 0 returns -1
cpr_save_fd pc.bios, id 0, fd 22
qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 22 host
0x7fec18e00000
cpr_find_fd pc.rom, id 0 returns -1
cpr_save_fd pc.rom, id 0, fd 23
qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 23 host
0x7fec18c00000
cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns -1
cpr_save_fd 0000:00:01.0/e1000e.rom, id 0, fd 24
qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size
262144 fd 24 host 0x7fec18a00000
cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns -1
cpr_save_fd 0000:00:02.0/vga.vram, id 0, fd 25
qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size
67108864 fd 25 host 0x7feb77e00000
cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.vrom, id 0, fd 27
qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192
fd 27 host 0x7fec18800000
cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.vram, id 0, fd 28
qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size
67108864 fd 28 host 0x7feb73c00000
cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.rom, id 0, fd 34
qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536
fd 34 host 0x7fec18600000
cpr_find_fd /rom@etc/acpi/tables, id 0 returns -1
cpr_save_fd /rom@etc/acpi/tables, id 0, fd 35
qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size
2097152 fd 35 host 0x7fec18200000
cpr_find_fd /rom@etc/table-loader, id 0 returns -1
cpr_save_fd /rom@etc/table-loader, id 0, fd 36
qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536
fd 36 host 0x7feb8b600000
cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns -1
cpr_save_fd /rom@etc/acpi/rsdp, id 0, fd 37
qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd
37 host 0x7feb8b400000

cpr_state_save cpr-transfer mode
cpr_transfer_output /var/run/alma8cpr-dst.sock
Target:
cpr_transfer_input /var/run/alma8cpr-dst.sock
cpr_state_load cpr-transfer mode
cpr_find_fd pc.bios, id 0 returns 20
qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 20 host
0x7fcdc9800000
cpr_find_fd pc.rom, id 0 returns 19
qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 19 host
0x7fcdc9600000
cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns 18
qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size
262144 fd 18 host 0x7fcdc9400000
cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns 17
qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size
67108864 fd 17 host 0x7fcd27e00000
cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns 16
qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192
fd 16 host 0x7fcdc9200000
cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns 15
qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size
67108864 fd 15 host 0x7fcd23c00000
cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns 14
qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536
fd 14 host 0x7fcdc8800000
cpr_find_fd /rom@etc/acpi/tables, id 0 returns 13
qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size
2097152 fd 13 host 0x7fcdc8400000
cpr_find_fd /rom@etc/table-loader, id 0 returns 11
qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536
fd 11 host 0x7fcdc8200000
cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns 10
qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd
10 host 0x7fcd3be00000
Looks like both vga.vram and qxl.vram are being preserved (with the
same
addresses), and no incompatible ram blocks are found during migration.
Sorry, addressed are not the same, of course.  However corresponding
ram
blocks do seem to be preserved and initialized.
So far, I have not reproduced the guest driver failure.

However, I have isolated places where new QEMU improperly writes to
the qxl memory regions prior to starting the guest, by mmap'ing them
readonly after cpr:

    qemu_ram_alloc_internal()
      if (reused && (strstr(name, "qxl") || strstr("name", "vga")))
          ram_flags |= RAM_READONLY;
      new_block = qemu_ram_alloc_from_fd(...)

I have attached a draft fix; try it and let me know.
My console window looks fine before and after cpr, using
-vnc $hostip:0 -vga qxl

- Steve
Regarding the reproduce: when I launch the buggy version with the same
options as you, i.e. "-vnc 0.0.0.0:$port -vga qxl", and do cpr-transfer,
my VNC client silently hangs on the target after a while.  Could it
happen on your stand as well?
cpr does not preserve the vnc connection and session.  To test, I specify
port 0 for the source VM and port 1 for the dest.  When the src vnc goes
dormant the dest vnc becomes active.
Sure, I meant that VNC on the dest (on the port 1) works for a while
after the migration and then hangs, apparently after the guest QXL crash.
Could you try launching VM with
"-nographic -device qxl-vga"?  That way VM's serial console is given you
directly in the shell, so when qxl driver crashes you're still able to
inspect the kernel messages.
I have been running like that, but have not reproduced the qxl driver
crash,
and I suspect my guest image+kernel is too old.
Yes, that's probably the case.  But the crash occurs on my Fedora 41
guest with the 6.11.5-300.fc41.x86_64 kernel, so newer kernels seem to
be buggy.
However, once I realized the
issue was post-cpr modification of qxl memory, I switched my attention
to the
fix.
As for your patch, I can report that it doesn't resolve the issue as it
is.  But I was able to track down another possible memory corruption
using your approach with readonly mmap'ing:
Program terminated with signal SIGSEGV, Segmentation fault.
#0  init_qxl_ram (d=0x5638996e0e70) at ../hw/display/qxl.c:412
412         d->ram->magic       = cpu_to_le32(QXL_RAM_MAGIC);
[Current thread is 1 (Thread 0x7f1a4f83b480 (LWP 229798))]
(gdb) bt
#0  init_qxl_ram (d=0x5638996e0e70) at ../hw/display/qxl.c:412
#1  0x0000563896e7f467 in qxl_realize_common (qxl=0x5638996e0e70,
errp=0x7ffd3c2b8170) at ../hw/display/qxl.c:2142
#2  0x0000563896e7fda1 in qxl_realize_primary (dev=0x5638996e0e70,
errp=0x7ffd3c2b81d0) at ../hw/display/qxl.c:2257
#3  0x0000563896c7e8f2 in pci_qdev_realize (qdev=0x5638996e0e70,
errp=0x7ffd3c2b8250) at ../hw/pci/pci.c:2174
#4  0x00005638970eb54b in device_set_realized (obj=0x5638996e0e70,
value=true, errp=0x7ffd3c2b84e0) at ../hw/core/qdev.c:494
#5  0x00005638970f5e14 in property_set_bool (obj=0x5638996e0e70,
v=0x5638996f3770, name=0x56389759b141 "realized",
opaque=0x5638987893d0, errp=0x7ffd3c2b84e0)
      at ../qom/object.c:2374
#6  0x00005638970f39f8 in object_property_set (obj=0x5638996e0e70,
name=0x56389759b141 "realized", v=0x5638996f3770, errp=0x7ffd3c2b84e0)
      at ../qom/object.c:1449
#7  0x00005638970f8586 in object_property_set_qobject
(obj=0x5638996e0e70, name=0x56389759b141 "realized",
value=0x5638996df900, errp=0x7ffd3c2b84e0)
      at ../qom/qom-qobject.c:28
#8  0x00005638970f3d8d in object_property_set_bool
(obj=0x5638996e0e70, name=0x56389759b141 "realized", value=true,
errp=0x7ffd3c2b84e0)
      at ../qom/object.c:1519
#9  0x00005638970eacb0 in qdev_realize (dev=0x5638996e0e70,
bus=0x563898cf3c20, errp=0x7ffd3c2b84e0) at ../hw/core/qdev.c:276
#10 0x0000563896dba675 in qdev_device_add_from_qdict
(opts=0x5638996dfe50, from_json=false, errp=0x7ffd3c2b84e0) at ../
system/qdev-monitor.c:714
#11 0x0000563896dba721 in qdev_device_add (opts=0x563898786150,
errp=0x56389855dc40 <error_fatal>) at ../system/qdev-monitor.c:733
#12 0x0000563896dc48f1 in device_init_func (opaque=0x0,
opts=0x563898786150, errp=0x56389855dc40 <error_fatal>) at ../system/
vl.c:1207
#13 0x000056389737a6cc in qemu_opts_foreach
      (list=0x563898427b60 <qemu_device_opts>, func=0x563896dc48ca
<device_init_func>, opaque=0x0, errp=0x56389855dc40 <error_fatal>)
      at ../util/qemu-option.c:1135
#14 0x0000563896dc89b5 in qemu_create_cli_devices () at ../system/
vl.c:2745
#15 0x0000563896dc8c00 in qmp_x_exit_preconfig (errp=0x56389855dc40
<error_fatal>) at ../system/vl.c:2806
#16 0x0000563896dcb5de in qemu_init (argc=33, argv=0x7ffd3c2b8948)
at ../system/vl.c:3838
#17 0x0000563897297323 in main (argc=33, argv=0x7ffd3c2b8948) at ../
system/main.c:72
So the attached adjusted version of your patch does seem to help.  At
least I can't reproduce the crash on my stand.
Thanks for the stack trace; the calls to SPICE_RING_INIT in init_qxl_ram
are
definitely harmful.  Try V2 of the patch, attached, which skips the lines
of init_qxl_ram that modify guest memory.
Thanks, your v2 patch does seem to prevent the crash.  Would you re-send
it to the list as a proper fix?
I'm wondering, could it be useful to explicitly mark all the reused
memory regions readonly upon cpr-transfer, and then make them writable
back again after the migration is done?  That way we will be segfaulting
early on instead of debugging tricky memory corruptions.
It's a useful debugging technique, but changing protection on a large
memory region
can be too expensive for production due to TLB shootdowns.

Also, there are cases where writes are performed but the value is
guaranteed to
be the same:
   qxl_post_load()
     qxl_set_mode()
       d->rom->mode = cpu_to_le32(modenr);
The value is the same because mode and shadow_rom.mode were passed in
vmstate
from old qemu.
There're also cases where devices' ROM might be re-initialized.  E.g.
this segfault occures upon further exploration of RO mapped RAM blocks:
Program terminated with signal SIGSEGV, Segmentation fault.
#0  __memmove_avx_unaligned_erms () at 
../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:664
664             rep     movsb
[Current thread is 1 (Thread 0x7f6e7d08b480 (LWP 310379))]
(gdb) bt
#0  __memmove_avx_unaligned_erms () at 
../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:664
#1  0x000055aa1d030ecd in rom_set_mr (rom=0x55aa200ba380, owner=0x55aa2019ac10, 
name=0x7fffb8272bc0 "/rom@etc/acpi/tables", ro=true)
     at ../hw/core/loader.c:1032
#2  0x000055aa1d031577 in rom_add_blob
     (name=0x55aa1da51f13 "etc/acpi/tables", blob=0x55aa208a1070, len=131072, max_len=2097152, 
addr=18446744073709551615, fw_file_name=0x55aa1da51f13 "etc/acpi/tables", 
fw_callback=0x55aa1d441f59 <acpi_build_update>, callback_opaque=0x55aa20ff0010, as=0x0, 
read_only=true) at ../hw/core/loader.c:1147
#3  0x000055aa1cfd788d in acpi_add_rom_blob
     (update=0x55aa1d441f59 <acpi_build_update>, opaque=0x55aa20ff0010, 
blob=0x55aa1fc9aa00, name=0x55aa1da51f13 "etc/acpi/tables") at ../hw/acpi/utils.c:46
#4  0x000055aa1d44213f in acpi_setup () at ../hw/i386/acpi-build.c:2720
#5  0x000055aa1d434199 in pc_machine_done (notifier=0x55aa1ff15050, data=0x0) 
at ../hw/i386/pc.c:638
#6  0x000055aa1d876845 in notifier_list_notify (list=0x55aa1ea25c10 
<machine_init_done_notifiers>, data=0x0) at ../util/notify.c:39
#7  0x000055aa1d039ee5 in qdev_machine_creation_done () at 
../hw/core/machine.c:1749
#8  0x000055aa1d2c7b3e in qemu_machine_creation_done (errp=0x55aa1ea5cc40 
<error_fatal>) at ../system/vl.c:2779
#9  0x000055aa1d2c7c7d in qmp_x_exit_preconfig (errp=0x55aa1ea5cc40 
<error_fatal>) at ../system/vl.c:2807
#10 0x000055aa1d2ca64f in qemu_init (argc=35, argv=0x7fffb82730e8) at 
../system/vl.c:3838
#11 0x000055aa1d79638c in main (argc=35, argv=0x7fffb82730e8) at 
../system/main.c:72
I'm not sure whether ACPI tables ROM in particular is rewritten with the
same content, but there might be cases where ROM can be read from file
system upon initialization.  That is undesirable as guest kernel
certainly won't be too happy about sudden change of the device's ROM
content.

So the issue we're dealing with here is any unwanted memory related
device initialization upon cpr.

For now the only thing that comes to my mind is to make a test where we
put as many devices as we can into a VM, make ram blocks RO upon cpr
(and remap them as RW later after migration is done, if needed), and
catch any unwanted memory violations.  As Den suggested, we might
consider adding that behaviour as a separate non-default option (or
"migrate" command flag specific to cpr-transfer), which would only be
used in the testing.

Andrey
No way. ACPI with the source must be used in the same way as BIOSes
and optional ROMs.

Den

On 3/6/2025 10:52 AM, Denis V. Lunev wrote:
On 3/6/25 16:16, Andrey Drobyshev wrote:
On 3/5/25 11:19 PM, Steven Sistare wrote:
On 3/5/2025 11:50 AM, Andrey Drobyshev wrote:
On 3/4/25 9:05 PM, Steven Sistare wrote:
On 2/28/2025 1:37 PM, Andrey Drobyshev wrote:
On 2/28/25 8:35 PM, Andrey Drobyshev wrote:
On 2/28/25 8:20 PM, Steven Sistare wrote:
On 2/28/2025 1:13 PM, Steven Sistare wrote:
On 2/28/2025 12:39 PM, Andrey Drobyshev wrote:
Hi all,

We've been experimenting with cpr-transfer migration mode recently
and
have discovered the following issue with the guest QXL driver:

Run migration source:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-src.sock

$EMULATOR -enable-kvm \
        -machine q35 \
        -cpu host -smp 2 -m 2G \
        -object memory-backend-file,id=ram0,size=2G,mem-path=/
dev/shm/
ram0,share=on\
        -machine memory-backend=ram0 \
        -machine aux-ram-share=on \
        -drive file=$ROOTFS,media=disk,if=virtio \
        -qmp unix:$QMPSOCK,server=on,wait=off \
        -nographic \
        -device qxl-vga
Run migration target:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-dst.sock
$EMULATOR -enable-kvm \
        -machine q35 \
        -cpu host -smp 2 -m 2G \
        -object memory-backend-file,id=ram0,size=2G,mem-path=/
dev/shm/
ram0,share=on\
        -machine memory-backend=ram0 \
        -machine aux-ram-share=on \
        -drive file=$ROOTFS,media=disk,if=virtio \
        -qmp unix:$QMPSOCK,server=on,wait=off \
        -nographic \
        -device qxl-vga \
        -incoming tcp:0:44444 \
        -incoming '{"channel-type": "cpr", "addr": { "transport":
"socket", "type": "unix", "path": "/var/run/alma8cpr-dst.sock"}}'
Launch the migration:
QMPSHELL=/root/src/qemu/master/scripts/qmp/qmp-shell
QMPSOCK=/var/run/alma8qmp-src.sock

$QMPSHELL -p $QMPSOCK <<EOF
        migrate-set-parameters mode=cpr-transfer
        migrate channels=[{"channel-type":"main","addr":
{"transport":"socket","type":"inet","host":"0","port":"44444"}},
{"channel-type":"cpr","addr":
{"transport":"socket","type":"unix","path":"/var/run/alma8cpr-
dst.sock"}}]
EOF
Then, after a while, QXL guest driver on target crashes spewing the
following messages:
[   73.962002] [TTM] Buffer eviction failed
[   73.962072] qxl 0000:00:02.0: object_init failed for (3149824,
0x00000001)
[   73.962081] [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to
allocate VRAM BO
That seems to be a known kernel QXL driver bug:
https://lore.kernel.org/all/20220907094423.93581-1-
min_halo@163.com/T/
https://lore.kernel.org/lkml/ZTgydqRlK6WX_b29@eldamar.lan/
(the latter discussion contains that reproduce script which
speeds up
the crash in the guest):
#!/bin/bash

chvt 3

for j in $(seq 80); do
            echo "$(date) starting round $j"
            if [ "$(journalctl --boot | grep "failed to allocate
VRAM
BO")" != "" ]; then
                    echo "bug was reproduced after $j tries"
                    exit 1
            fi
            for i in $(seq 100); do
                    dmesg > /dev/tty3
            done
done

echo "bug could not be reproduced"
exit 0
The bug itself seems to remain unfixed, as I was able to reproduce
that
with Fedora 41 guest, as well as AlmaLinux 8 guest. However our
cpr-transfer code also seems to be buggy as it triggers the crash -
without the cpr-transfer migration the above reproduce doesn't
lead to
crash on the source VM.

I suspect that, as cpr-transfer doesn't migrate the guest
memory, but
rather passes it through the memory backend object, our code might
somehow corrupt the VRAM.  However, I wasn't able to trace the
corruption so far.

Could somebody help the investigation and take a look into
this?  Any
suggestions would be appreciated.  Thanks!
Possibly some memory region created by qxl is not being preserved.
Try adding these traces to see what is preserved:

-trace enable='*cpr*'
-trace enable='*ram_alloc*'
Also try adding this patch to see if it flags any ram blocks as not
compatible with cpr.  A message is printed at migration start time.
   Â
https://lore.kernel.org/qemu-devel/1740667681-257312-1-git-send-
email-
steven.sistare@oracle.com/

- Steve
With the traces enabled + the "migration: ram block cpr blockers"
patch
applied:

Source:
cpr_find_fd pc.bios, id 0 returns -1
cpr_save_fd pc.bios, id 0, fd 22
qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 22 host
0x7fec18e00000
cpr_find_fd pc.rom, id 0 returns -1
cpr_save_fd pc.rom, id 0, fd 23
qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 23 host
0x7fec18c00000
cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns -1
cpr_save_fd 0000:00:01.0/e1000e.rom, id 0, fd 24
qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size
262144 fd 24 host 0x7fec18a00000
cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns -1
cpr_save_fd 0000:00:02.0/vga.vram, id 0, fd 25
qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size
67108864 fd 25 host 0x7feb77e00000
cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.vrom, id 0, fd 27
qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192
fd 27 host 0x7fec18800000
cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.vram, id 0, fd 28
qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size
67108864 fd 28 host 0x7feb73c00000
cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.rom, id 0, fd 34
qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536
fd 34 host 0x7fec18600000
cpr_find_fd /rom@etc/acpi/tables, id 0 returns -1
cpr_save_fd /rom@etc/acpi/tables, id 0, fd 35
qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size
2097152 fd 35 host 0x7fec18200000
cpr_find_fd /rom@etc/table-loader, id 0 returns -1
cpr_save_fd /rom@etc/table-loader, id 0, fd 36
qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536
fd 36 host 0x7feb8b600000
cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns -1
cpr_save_fd /rom@etc/acpi/rsdp, id 0, fd 37
qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd
37 host 0x7feb8b400000

cpr_state_save cpr-transfer mode
cpr_transfer_output /var/run/alma8cpr-dst.sock
Target:
cpr_transfer_input /var/run/alma8cpr-dst.sock
cpr_state_load cpr-transfer mode
cpr_find_fd pc.bios, id 0 returns 20
qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 20 host
0x7fcdc9800000
cpr_find_fd pc.rom, id 0 returns 19
qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 19 host
0x7fcdc9600000
cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns 18
qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size
262144 fd 18 host 0x7fcdc9400000
cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns 17
qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size
67108864 fd 17 host 0x7fcd27e00000
cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns 16
qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192
fd 16 host 0x7fcdc9200000
cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns 15
qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size
67108864 fd 15 host 0x7fcd23c00000
cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns 14
qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536
fd 14 host 0x7fcdc8800000
cpr_find_fd /rom@etc/acpi/tables, id 0 returns 13
qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size
2097152 fd 13 host 0x7fcdc8400000
cpr_find_fd /rom@etc/table-loader, id 0 returns 11
qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536
fd 11 host 0x7fcdc8200000
cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns 10
qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd
10 host 0x7fcd3be00000
Looks like both vga.vram and qxl.vram are being preserved (with the
same
addresses), and no incompatible ram blocks are found during migration.
Sorry, addressed are not the same, of course.  However corresponding
ram
blocks do seem to be preserved and initialized.
So far, I have not reproduced the guest driver failure.

However, I have isolated places where new QEMU improperly writes to
the qxl memory regions prior to starting the guest, by mmap'ing them
readonly after cpr:

    qemu_ram_alloc_internal()
      if (reused && (strstr(name, "qxl") || strstr("name", "vga")))
          ram_flags |= RAM_READONLY;
      new_block = qemu_ram_alloc_from_fd(...)

I have attached a draft fix; try it and let me know.
My console window looks fine before and after cpr, using
-vnc $hostip:0 -vga qxl

- Steve
Regarding the reproduce: when I launch the buggy version with the same
options as you, i.e. "-vnc 0.0.0.0:$port -vga qxl", and do cpr-transfer,
my VNC client silently hangs on the target after a while.  Could it
happen on your stand as well?
cpr does not preserve the vnc connection and session.  To test, I specify
port 0 for the source VM and port 1 for the dest.  When the src vnc goes
dormant the dest vnc becomes active.
Sure, I meant that VNC on the dest (on the port 1) works for a while
after the migration and then hangs, apparently after the guest QXL crash.
Could you try launching VM with
"-nographic -device qxl-vga"?  That way VM's serial console is given you
directly in the shell, so when qxl driver crashes you're still able to
inspect the kernel messages.
I have been running like that, but have not reproduced the qxl driver
crash,
and I suspect my guest image+kernel is too old.
Yes, that's probably the case.  But the crash occurs on my Fedora 41
guest with the 6.11.5-300.fc41.x86_64 kernel, so newer kernels seem to
be buggy.
However, once I realized the
issue was post-cpr modification of qxl memory, I switched my attention
to the
fix.
As for your patch, I can report that it doesn't resolve the issue as it
is.  But I was able to track down another possible memory corruption
using your approach with readonly mmap'ing:
Program terminated with signal SIGSEGV, Segmentation fault.
#0  init_qxl_ram (d=0x5638996e0e70) at ../hw/display/qxl.c:412
412         d->ram->magic       = cpu_to_le32(QXL_RAM_MAGIC);
[Current thread is 1 (Thread 0x7f1a4f83b480 (LWP 229798))]
(gdb) bt
#0  init_qxl_ram (d=0x5638996e0e70) at ../hw/display/qxl.c:412
#1  0x0000563896e7f467 in qxl_realize_common (qxl=0x5638996e0e70,
errp=0x7ffd3c2b8170) at ../hw/display/qxl.c:2142
#2  0x0000563896e7fda1 in qxl_realize_primary (dev=0x5638996e0e70,
errp=0x7ffd3c2b81d0) at ../hw/display/qxl.c:2257
#3  0x0000563896c7e8f2 in pci_qdev_realize (qdev=0x5638996e0e70,
errp=0x7ffd3c2b8250) at ../hw/pci/pci.c:2174
#4  0x00005638970eb54b in device_set_realized (obj=0x5638996e0e70,
value=true, errp=0x7ffd3c2b84e0) at ../hw/core/qdev.c:494
#5  0x00005638970f5e14 in property_set_bool (obj=0x5638996e0e70,
v=0x5638996f3770, name=0x56389759b141 "realized",
opaque=0x5638987893d0, errp=0x7ffd3c2b84e0)
      at ../qom/object.c:2374
#6  0x00005638970f39f8 in object_property_set (obj=0x5638996e0e70,
name=0x56389759b141 "realized", v=0x5638996f3770, errp=0x7ffd3c2b84e0)
      at ../qom/object.c:1449
#7  0x00005638970f8586 in object_property_set_qobject
(obj=0x5638996e0e70, name=0x56389759b141 "realized",
value=0x5638996df900, errp=0x7ffd3c2b84e0)
      at ../qom/qom-qobject.c:28
#8  0x00005638970f3d8d in object_property_set_bool
(obj=0x5638996e0e70, name=0x56389759b141 "realized", value=true,
errp=0x7ffd3c2b84e0)
      at ../qom/object.c:1519
#9  0x00005638970eacb0 in qdev_realize (dev=0x5638996e0e70,
bus=0x563898cf3c20, errp=0x7ffd3c2b84e0) at ../hw/core/qdev.c:276
#10 0x0000563896dba675 in qdev_device_add_from_qdict
(opts=0x5638996dfe50, from_json=false, errp=0x7ffd3c2b84e0) at ../
system/qdev-monitor.c:714
#11 0x0000563896dba721 in qdev_device_add (opts=0x563898786150,
errp=0x56389855dc40 <error_fatal>) at ../system/qdev-monitor.c:733
#12 0x0000563896dc48f1 in device_init_func (opaque=0x0,
opts=0x563898786150, errp=0x56389855dc40 <error_fatal>) at ../system/
vl.c:1207
#13 0x000056389737a6cc in qemu_opts_foreach
      (list=0x563898427b60 <qemu_device_opts>, func=0x563896dc48ca
<device_init_func>, opaque=0x0, errp=0x56389855dc40 <error_fatal>)
      at ../util/qemu-option.c:1135
#14 0x0000563896dc89b5 in qemu_create_cli_devices () at ../system/
vl.c:2745
#15 0x0000563896dc8c00 in qmp_x_exit_preconfig (errp=0x56389855dc40
<error_fatal>) at ../system/vl.c:2806
#16 0x0000563896dcb5de in qemu_init (argc=33, argv=0x7ffd3c2b8948)
at ../system/vl.c:3838
#17 0x0000563897297323 in main (argc=33, argv=0x7ffd3c2b8948) at ../
system/main.c:72
So the attached adjusted version of your patch does seem to help.  At
least I can't reproduce the crash on my stand.
Thanks for the stack trace; the calls to SPICE_RING_INIT in init_qxl_ram
are
definitely harmful.  Try V2 of the patch, attached, which skips the lines
of init_qxl_ram that modify guest memory.
Thanks, your v2 patch does seem to prevent the crash.  Would you re-send
it to the list as a proper fix?
Yes.  Was waiting for your confirmation.
I'm wondering, could it be useful to explicitly mark all the reused
memory regions readonly upon cpr-transfer, and then make them writable
back again after the migration is done?  That way we will be segfaulting
early on instead of debugging tricky memory corruptions.
It's a useful debugging technique, but changing protection on a large
memory region
can be too expensive for production due to TLB shootdowns.

Also, there are cases where writes are performed but the value is
guaranteed to
be the same:
   qxl_post_load()
     qxl_set_mode()
       d->rom->mode = cpu_to_le32(modenr);
The value is the same because mode and shadow_rom.mode were passed in
vmstate
from old qemu.
There're also cases where devices' ROM might be re-initialized.  E.g.
this segfault occures upon further exploration of RO mapped RAM blocks:
Program terminated with signal SIGSEGV, Segmentation fault.
#0  __memmove_avx_unaligned_erms () at 
../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:664
664             rep     movsb
[Current thread is 1 (Thread 0x7f6e7d08b480 (LWP 310379))]
(gdb) bt
#0  __memmove_avx_unaligned_erms () at 
../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:664
#1  0x000055aa1d030ecd in rom_set_mr (rom=0x55aa200ba380, owner=0x55aa2019ac10, 
name=0x7fffb8272bc0 "/rom@etc/acpi/tables", ro=true)
     at ../hw/core/loader.c:1032
#2  0x000055aa1d031577 in rom_add_blob
     (name=0x55aa1da51f13 "etc/acpi/tables", blob=0x55aa208a1070, len=131072, max_len=2097152, 
addr=18446744073709551615, fw_file_name=0x55aa1da51f13 "etc/acpi/tables", 
fw_callback=0x55aa1d441f59 <acpi_build_update>, callback_opaque=0x55aa20ff0010, as=0x0, 
read_only=true) at ../hw/core/loader.c:1147
#3  0x000055aa1cfd788d in acpi_add_rom_blob
     (update=0x55aa1d441f59 <acpi_build_update>, opaque=0x55aa20ff0010, 
blob=0x55aa1fc9aa00, name=0x55aa1da51f13 "etc/acpi/tables") at ../hw/acpi/utils.c:46
#4  0x000055aa1d44213f in acpi_setup () at ../hw/i386/acpi-build.c:2720
#5  0x000055aa1d434199 in pc_machine_done (notifier=0x55aa1ff15050, data=0x0) 
at ../hw/i386/pc.c:638
#6  0x000055aa1d876845 in notifier_list_notify (list=0x55aa1ea25c10 
<machine_init_done_notifiers>, data=0x0) at ../util/notify.c:39
#7  0x000055aa1d039ee5 in qdev_machine_creation_done () at 
../hw/core/machine.c:1749
#8  0x000055aa1d2c7b3e in qemu_machine_creation_done (errp=0x55aa1ea5cc40 
<error_fatal>) at ../system/vl.c:2779
#9  0x000055aa1d2c7c7d in qmp_x_exit_preconfig (errp=0x55aa1ea5cc40 
<error_fatal>) at ../system/vl.c:2807
#10 0x000055aa1d2ca64f in qemu_init (argc=35, argv=0x7fffb82730e8) at 
../system/vl.c:3838
#11 0x000055aa1d79638c in main (argc=35, argv=0x7fffb82730e8) at 
../system/main.c:72
I'm not sure whether ACPI tables ROM in particular is rewritten with the
same content, but there might be cases where ROM can be read from file
system upon initialization.  That is undesirable as guest kernel
certainly won't be too happy about sudden change of the device's ROM
content.

So the issue we're dealing with here is any unwanted memory related
device initialization upon cpr.

For now the only thing that comes to my mind is to make a test where we
put as many devices as we can into a VM, make ram blocks RO upon cpr
(and remap them as RW later after migration is done, if needed), and
catch any unwanted memory violations.  As Den suggested, we might
consider adding that behaviour as a separate non-default option (or
"migrate" command flag specific to cpr-transfer), which would only be
used in the testing.
I'll look into adding an option, but there may be too many false positives,
such as the qxl_set_mode case above.  And the maintainers may object to me
eliminating the false positives by adding more CPR_IN tests, due to gratuitous
(from their POV) ugliness.

But I will use the technique to look for more write violations.
Andrey
No way. ACPI with the source must be used in the same way as BIOSes
and optional ROMs.
Yup, its a bug.  Will fix.

- Steve

see
1741380954-341079-1-git-send-email-steven.sistare@oracle.com
/">https://lore.kernel.org/qemu-devel/
1741380954-341079-1-git-send-email-steven.sistare@oracle.com
/
- Steve

On 3/6/2025 11:13 AM, Steven Sistare wrote:
On 3/6/2025 10:52 AM, Denis V. Lunev wrote:
On 3/6/25 16:16, Andrey Drobyshev wrote:
On 3/5/25 11:19 PM, Steven Sistare wrote:
On 3/5/2025 11:50 AM, Andrey Drobyshev wrote:
On 3/4/25 9:05 PM, Steven Sistare wrote:
On 2/28/2025 1:37 PM, Andrey Drobyshev wrote:
On 2/28/25 8:35 PM, Andrey Drobyshev wrote:
On 2/28/25 8:20 PM, Steven Sistare wrote:
On 2/28/2025 1:13 PM, Steven Sistare wrote:
On 2/28/2025 12:39 PM, Andrey Drobyshev wrote:
Hi all,

We've been experimenting with cpr-transfer migration mode recently
and
have discovered the following issue with the guest QXL driver:

Run migration source:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-src.sock

$EMULATOR -enable-kvm \
        -machine q35 \
        -cpu host -smp 2 -m 2G \
        -object memory-backend-file,id=ram0,size=2G,mem-path=/
dev/shm/
ram0,share=on\
        -machine memory-backend=ram0 \
        -machine aux-ram-share=on \
        -drive file=$ROOTFS,media=disk,if=virtio \
        -qmp unix:$QMPSOCK,server=on,wait=off \
        -nographic \
        -device qxl-vga
Run migration target:
EMULATOR=/path/to/emulator
ROOTFS=/path/to/image
QMPSOCK=/var/run/alma8qmp-dst.sock
$EMULATOR -enable-kvm \
        -machine q35 \
        -cpu host -smp 2 -m 2G \
        -object memory-backend-file,id=ram0,size=2G,mem-path=/
dev/shm/
ram0,share=on\
        -machine memory-backend=ram0 \
        -machine aux-ram-share=on \
        -drive file=$ROOTFS,media=disk,if=virtio \
        -qmp unix:$QMPSOCK,server=on,wait=off \
        -nographic \
        -device qxl-vga \
        -incoming tcp:0:44444 \
        -incoming '{"channel-type": "cpr", "addr": { "transport":
"socket", "type": "unix", "path": "/var/run/alma8cpr-dst.sock"}}'
Launch the migration:
QMPSHELL=/root/src/qemu/master/scripts/qmp/qmp-shell
QMPSOCK=/var/run/alma8qmp-src.sock

$QMPSHELL -p $QMPSOCK <<EOF
        migrate-set-parameters mode=cpr-transfer
        migrate channels=[{"channel-type":"main","addr":
{"transport":"socket","type":"inet","host":"0","port":"44444"}},
{"channel-type":"cpr","addr":
{"transport":"socket","type":"unix","path":"/var/run/alma8cpr-
dst.sock"}}]
EOF
Then, after a while, QXL guest driver on target crashes spewing the
following messages:
[   73.962002] [TTM] Buffer eviction failed
[   73.962072] qxl 0000:00:02.0: object_init failed for (3149824,
0x00000001)
[   73.962081] [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to
allocate VRAM BO
That seems to be a known kernel QXL driver bug:
https://lore.kernel.org/all/20220907094423.93581-1-
min_halo@163.com/T/
https://lore.kernel.org/lkml/ZTgydqRlK6WX_b29@eldamar.lan/
(the latter discussion contains that reproduce script which
speeds up
the crash in the guest):
#!/bin/bash

chvt 3

for j in $(seq 80); do
            echo "$(date) starting round $j"
            if [ "$(journalctl --boot | grep "failed to allocate
VRAM
BO")" != "" ]; then
                    echo "bug was reproduced after $j tries"
                    exit 1
            fi
            for i in $(seq 100); do
                    dmesg > /dev/tty3
            done
done

echo "bug could not be reproduced"
exit 0
The bug itself seems to remain unfixed, as I was able to reproduce
that
with Fedora 41 guest, as well as AlmaLinux 8 guest. However our
cpr-transfer code also seems to be buggy as it triggers the crash -
without the cpr-transfer migration the above reproduce doesn't
lead to
crash on the source VM.

I suspect that, as cpr-transfer doesn't migrate the guest
memory, but
rather passes it through the memory backend object, our code might
somehow corrupt the VRAM.  However, I wasn't able to trace the
corruption so far.

Could somebody help the investigation and take a look into
this?  Any
suggestions would be appreciated.  Thanks!
Possibly some memory region created by qxl is not being preserved.
Try adding these traces to see what is preserved:

-trace enable='*cpr*'
-trace enable='*ram_alloc*'
Also try adding this patch to see if it flags any ram blocks as not
compatible with cpr.  A message is printed at migration start time.
   Â
https://lore.kernel.org/qemu-devel/1740667681-257312-1-git-send-
email-
steven.sistare@oracle.com/

- Steve
With the traces enabled + the "migration: ram block cpr blockers"
patch
applied:

Source:
cpr_find_fd pc.bios, id 0 returns -1
cpr_save_fd pc.bios, id 0, fd 22
qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 22 host
0x7fec18e00000
cpr_find_fd pc.rom, id 0 returns -1
cpr_save_fd pc.rom, id 0, fd 23
qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 23 host
0x7fec18c00000
cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns -1
cpr_save_fd 0000:00:01.0/e1000e.rom, id 0, fd 24
qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size
262144 fd 24 host 0x7fec18a00000
cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns -1
cpr_save_fd 0000:00:02.0/vga.vram, id 0, fd 25
qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size
67108864 fd 25 host 0x7feb77e00000
cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.vrom, id 0, fd 27
qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192
fd 27 host 0x7fec18800000
cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.vram, id 0, fd 28
qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size
67108864 fd 28 host 0x7feb73c00000
cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns -1
cpr_save_fd 0000:00:02.0/qxl.rom, id 0, fd 34
qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536
fd 34 host 0x7fec18600000
cpr_find_fd /rom@etc/acpi/tables, id 0 returns -1
cpr_save_fd /rom@etc/acpi/tables, id 0, fd 35
qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size
2097152 fd 35 host 0x7fec18200000
cpr_find_fd /rom@etc/table-loader, id 0 returns -1
cpr_save_fd /rom@etc/table-loader, id 0, fd 36
qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536
fd 36 host 0x7feb8b600000
cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns -1
cpr_save_fd /rom@etc/acpi/rsdp, id 0, fd 37
qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd
37 host 0x7feb8b400000

cpr_state_save cpr-transfer mode
cpr_transfer_output /var/run/alma8cpr-dst.sock
Target:
cpr_transfer_input /var/run/alma8cpr-dst.sock
cpr_state_load cpr-transfer mode
cpr_find_fd pc.bios, id 0 returns 20
qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 20 host
0x7fcdc9800000
cpr_find_fd pc.rom, id 0 returns 19
qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 19 host
0x7fcdc9600000
cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns 18
qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size
262144 fd 18 host 0x7fcdc9400000
cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns 17
qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size
67108864 fd 17 host 0x7fcd27e00000
cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns 16
qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192
fd 16 host 0x7fcdc9200000
cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns 15
qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size
67108864 fd 15 host 0x7fcd23c00000
cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns 14
qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536
fd 14 host 0x7fcdc8800000
cpr_find_fd /rom@etc/acpi/tables, id 0 returns 13
qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size
2097152 fd 13 host 0x7fcdc8400000
cpr_find_fd /rom@etc/table-loader, id 0 returns 11
qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536
fd 11 host 0x7fcdc8200000
cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns 10
qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd
10 host 0x7fcd3be00000
Looks like both vga.vram and qxl.vram are being preserved (with the
same
addresses), and no incompatible ram blocks are found during migration.
Sorry, addressed are not the same, of course.  However corresponding
ram
blocks do seem to be preserved and initialized.
So far, I have not reproduced the guest driver failure.

However, I have isolated places where new QEMU improperly writes to
the qxl memory regions prior to starting the guest, by mmap'ing them
readonly after cpr:

    qemu_ram_alloc_internal()
      if (reused && (strstr(name, "qxl") || strstr("name", "vga")))
          ram_flags |= RAM_READONLY;
      new_block = qemu_ram_alloc_from_fd(...)

I have attached a draft fix; try it and let me know.
My console window looks fine before and after cpr, using
-vnc $hostip:0 -vga qxl

- Steve
Regarding the reproduce: when I launch the buggy version with the same
options as you, i.e. "-vnc 0.0.0.0:$port -vga qxl", and do cpr-transfer,
my VNC client silently hangs on the target after a while.  Could it
happen on your stand as well?
cpr does not preserve the vnc connection and session.  To test, I specify
port 0 for the source VM and port 1 for the dest.  When the src vnc goes
dormant the dest vnc becomes active.
Sure, I meant that VNC on the dest (on the port 1) works for a while
after the migration and then hangs, apparently after the guest QXL crash.
Could you try launching VM with
"-nographic -device qxl-vga"?  That way VM's serial console is given you
directly in the shell, so when qxl driver crashes you're still able to
inspect the kernel messages.
I have been running like that, but have not reproduced the qxl driver
crash,
and I suspect my guest image+kernel is too old.
Yes, that's probably the case.  But the crash occurs on my Fedora 41
guest with the 6.11.5-300.fc41.x86_64 kernel, so newer kernels seem to
be buggy.
However, once I realized the
issue was post-cpr modification of qxl memory, I switched my attention
to the
fix.
As for your patch, I can report that it doesn't resolve the issue as it
is.  But I was able to track down another possible memory corruption
using your approach with readonly mmap'ing:
Program terminated with signal SIGSEGV, Segmentation fault.
#0  init_qxl_ram (d=0x5638996e0e70) at ../hw/display/qxl.c:412
412         d->ram->magic       = cpu_to_le32(QXL_RAM_MAGIC);
[Current thread is 1 (Thread 0x7f1a4f83b480 (LWP 229798))]
(gdb) bt
#0  init_qxl_ram (d=0x5638996e0e70) at ../hw/display/qxl.c:412
#1  0x0000563896e7f467 in qxl_realize_common (qxl=0x5638996e0e70,
errp=0x7ffd3c2b8170) at ../hw/display/qxl.c:2142
#2  0x0000563896e7fda1 in qxl_realize_primary (dev=0x5638996e0e70,
errp=0x7ffd3c2b81d0) at ../hw/display/qxl.c:2257
#3  0x0000563896c7e8f2 in pci_qdev_realize (qdev=0x5638996e0e70,
errp=0x7ffd3c2b8250) at ../hw/pci/pci.c:2174
#4  0x00005638970eb54b in device_set_realized (obj=0x5638996e0e70,
value=true, errp=0x7ffd3c2b84e0) at ../hw/core/qdev.c:494
#5  0x00005638970f5e14 in property_set_bool (obj=0x5638996e0e70,
v=0x5638996f3770, name=0x56389759b141 "realized",
opaque=0x5638987893d0, errp=0x7ffd3c2b84e0)
      at ../qom/object.c:2374
#6  0x00005638970f39f8 in object_property_set (obj=0x5638996e0e70,
name=0x56389759b141 "realized", v=0x5638996f3770, errp=0x7ffd3c2b84e0)
      at ../qom/object.c:1449
#7  0x00005638970f8586 in object_property_set_qobject
(obj=0x5638996e0e70, name=0x56389759b141 "realized",
value=0x5638996df900, errp=0x7ffd3c2b84e0)
      at ../qom/qom-qobject.c:28
#8  0x00005638970f3d8d in object_property_set_bool
(obj=0x5638996e0e70, name=0x56389759b141 "realized", value=true,
errp=0x7ffd3c2b84e0)
      at ../qom/object.c:1519
#9  0x00005638970eacb0 in qdev_realize (dev=0x5638996e0e70,
bus=0x563898cf3c20, errp=0x7ffd3c2b84e0) at ../hw/core/qdev.c:276
#10 0x0000563896dba675 in qdev_device_add_from_qdict
(opts=0x5638996dfe50, from_json=false, errp=0x7ffd3c2b84e0) at ../
system/qdev-monitor.c:714
#11 0x0000563896dba721 in qdev_device_add (opts=0x563898786150,
errp=0x56389855dc40 <error_fatal>) at ../system/qdev-monitor.c:733
#12 0x0000563896dc48f1 in device_init_func (opaque=0x0,
opts=0x563898786150, errp=0x56389855dc40 <error_fatal>) at ../system/
vl.c:1207
#13 0x000056389737a6cc in qemu_opts_foreach
      (list=0x563898427b60 <qemu_device_opts>, func=0x563896dc48ca
<device_init_func>, opaque=0x0, errp=0x56389855dc40 <error_fatal>)
      at ../util/qemu-option.c:1135
#14 0x0000563896dc89b5 in qemu_create_cli_devices () at ../system/
vl.c:2745
#15 0x0000563896dc8c00 in qmp_x_exit_preconfig (errp=0x56389855dc40
<error_fatal>) at ../system/vl.c:2806
#16 0x0000563896dcb5de in qemu_init (argc=33, argv=0x7ffd3c2b8948)
at ../system/vl.c:3838
#17 0x0000563897297323 in main (argc=33, argv=0x7ffd3c2b8948) at ../
system/main.c:72
So the attached adjusted version of your patch does seem to help.  At
least I can't reproduce the crash on my stand.
Thanks for the stack trace; the calls to SPICE_RING_INIT in init_qxl_ram
are
definitely harmful.  Try V2 of the patch, attached, which skips the lines
of init_qxl_ram that modify guest memory.
Thanks, your v2 patch does seem to prevent the crash.  Would you re-send
it to the list as a proper fix?
Yes.  Was waiting for your confirmation.
I'm wondering, could it be useful to explicitly mark all the reused
memory regions readonly upon cpr-transfer, and then make them writable
back again after the migration is done?  That way we will be segfaulting
early on instead of debugging tricky memory corruptions.
It's a useful debugging technique, but changing protection on a large
memory region
can be too expensive for production due to TLB shootdowns.

Also, there are cases where writes are performed but the value is
guaranteed to
be the same:
   qxl_post_load()
     qxl_set_mode()
       d->rom->mode = cpu_to_le32(modenr);
The value is the same because mode and shadow_rom.mode were passed in
vmstate
from old qemu.
There're also cases where devices' ROM might be re-initialized.  E.g.
this segfault occures upon further exploration of RO mapped RAM blocks:
Program terminated with signal SIGSEGV, Segmentation fault.
#0  __memmove_avx_unaligned_erms () at 
../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:664
664             rep     movsb
[Current thread is 1 (Thread 0x7f6e7d08b480 (LWP 310379))]
(gdb) bt
#0  __memmove_avx_unaligned_erms () at 
../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:664
#1  0x000055aa1d030ecd in rom_set_mr (rom=0x55aa200ba380, owner=0x55aa2019ac10, 
name=0x7fffb8272bc0 "/rom@etc/acpi/tables", ro=true)
     at ../hw/core/loader.c:1032
#2  0x000055aa1d031577 in rom_add_blob
     (name=0x55aa1da51f13 "etc/acpi/tables", blob=0x55aa208a1070, len=131072, max_len=2097152, 
addr=18446744073709551615, fw_file_name=0x55aa1da51f13 "etc/acpi/tables", 
fw_callback=0x55aa1d441f59 <acpi_build_update>, callback_opaque=0x55aa20ff0010, as=0x0, 
read_only=true) at ../hw/core/loader.c:1147
#3  0x000055aa1cfd788d in acpi_add_rom_blob
     (update=0x55aa1d441f59 <acpi_build_update>, opaque=0x55aa20ff0010, 
blob=0x55aa1fc9aa00, name=0x55aa1da51f13 "etc/acpi/tables") at ../hw/acpi/utils.c:46
#4  0x000055aa1d44213f in acpi_setup () at ../hw/i386/acpi-build.c:2720
#5  0x000055aa1d434199 in pc_machine_done (notifier=0x55aa1ff15050, data=0x0) 
at ../hw/i386/pc.c:638
#6  0x000055aa1d876845 in notifier_list_notify (list=0x55aa1ea25c10 
<machine_init_done_notifiers>, data=0x0) at ../util/notify.c:39
#7  0x000055aa1d039ee5 in qdev_machine_creation_done () at 
../hw/core/machine.c:1749
#8  0x000055aa1d2c7b3e in qemu_machine_creation_done (errp=0x55aa1ea5cc40 
<error_fatal>) at ../system/vl.c:2779
#9  0x000055aa1d2c7c7d in qmp_x_exit_preconfig (errp=0x55aa1ea5cc40 
<error_fatal>) at ../system/vl.c:2807
#10 0x000055aa1d2ca64f in qemu_init (argc=35, argv=0x7fffb82730e8) at 
../system/vl.c:3838
#11 0x000055aa1d79638c in main (argc=35, argv=0x7fffb82730e8) at 
../system/main.c:72
I'm not sure whether ACPI tables ROM in particular is rewritten with the
same content, but there might be cases where ROM can be read from file
system upon initialization.  That is undesirable as guest kernel
certainly won't be too happy about sudden change of the device's ROM
content.

So the issue we're dealing with here is any unwanted memory related
device initialization upon cpr.

For now the only thing that comes to my mind is to make a test where we
put as many devices as we can into a VM, make ram blocks RO upon cpr
(and remap them as RW later after migration is done, if needed), and
catch any unwanted memory violations.  As Den suggested, we might
consider adding that behaviour as a separate non-default option (or
"migrate" command flag specific to cpr-transfer), which would only be
used in the testing.
I'll look into adding an option, but there may be too many false positives,
such as the qxl_set_mode case above.  And the maintainers may object to me
eliminating the false positives by adding more CPR_IN tests, due to gratuitous
(from their POV) ugliness.

But I will use the technique to look for more write violations.
Andrey
No way. ACPI with the source must be used in the same way as BIOSes
and optional ROMs.
Yup, its a bug.  Will fix.

- Steve