1 files changed, 720 insertions, 0 deletions

diff --git a/results/classifier/013/user-level/23270873 b/results/classifier/013/user-level/23270873
new file mode 100644
index 000000000..26df90e10
--- /dev/null
+++ b/results/classifier/013/user-level/23270873
@@ -0,0 +1,720 @@
+user-level: 0.896
+mistranslation: 0.881
+risc-v: 0.859
+operating system: 0.844
+boot: 0.830
+TCG: 0.828
+ppc: 0.827
+vnc: 0.820
+peripherals: 0.820
+device: 0.810
+hypervisor: 0.806
+KVM: 0.803
+virtual: 0.802
+permissions: 0.802
+register: 0.797
+VMM: 0.792
+debug: 0.788
+assembly: 0.768
+network: 0.768
+graphic: 0.764
+arm: 0.761
+socket: 0.758
+semantic: 0.752
+system: 0.748
+performance: 0.744
+architecture: 0.742
+kernel: 0.735
+PID: 0.731
+x86: 0.730
+files: 0.730
+alpha: 0.712
+i386: 0.705
+
+[Qemu-devel] [BUG?] aio_get_linux_aio: Assertion `ctx->linux_aio' failed
+
+Hi,
+
+I am seeing some strange QEMU assertion failures for qemu on s390x,
+which prevents a guest from starting.
+
+Git bisecting points to the following commit as the source of the error.
+
+commit ed6e2161715c527330f936d44af4c547f25f687e
+Author: Nishanth Aravamudan <address@hidden>
+Date:   Fri Jun 22 12:37:00 2018 -0700
+
+    linux-aio: properly bubble up errors from initialization
+
+    laio_init() can fail for a couple of reasons, which will lead to a NULL
+    pointer dereference in laio_attach_aio_context().
+
+    To solve this, add a aio_setup_linux_aio() function which is called
+    early in raw_open_common. If this fails, propagate the error up. The
+    signature of aio_get_linux_aio() was not modified, because it seems
+    preferable to return the actual errno from the possible failing
+    initialization calls.
+
+    Additionally, when the AioContext changes, we need to associate a
+    LinuxAioState with the new AioContext. Use the bdrv_attach_aio_context
+    callback and call the new aio_setup_linux_aio(), which will allocate a
+new AioContext if needed, and return errors on failures. If it
+fails for
+any reason, fallback to threaded AIO with an error message, as the
+    device is already in-use by the guest.
+
+    Add an assert that aio_get_linux_aio() cannot return NULL.
+
+    Signed-off-by: Nishanth Aravamudan <address@hidden>
+    Message-id: address@hidden
+    Signed-off-by: Stefan Hajnoczi <address@hidden>
+Not sure what is causing this assertion to fail. Here is the qemu
+command line of the guest, from qemu log, which throws this error:
+LC_ALL=C PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin
+QEMU_AUDIO_DRV=none /usr/local/bin/qemu-system-s390x -name
+guest=rt_vm1,debug-threads=on -S -object
+secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-21-rt_vm1/master-key.aes
+-machine s390-ccw-virtio-2.12,accel=kvm,usb=off,dump-guest-core=off -m
+1024 -realtime mlock=off -smp 4,sockets=4,cores=1,threads=1 -object
+iothread,id=iothread1 -uuid 0cde16cd-091d-41bd-9ac2-5243df5c9a0d
+-display none -no-user-config -nodefaults -chardev
+socket,id=charmonitor,fd=28,server,nowait -mon
+chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown
+-boot strict=on -drive
+file=/dev/mapper/360050763998b0883980000002a000031,format=raw,if=none,id=drive-virtio-disk0,cache=none,aio=native
+-device
+virtio-blk-ccw,iothread=iothread1,scsi=off,devno=fe.0.0001,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1,write-cache=on
+-netdev tap,fd=30,id=hostnet0,vhost=on,vhostfd=31 -device
+virtio-net-ccw,netdev=hostnet0,id=net0,mac=02:3a:c8:67:95:84,devno=fe.0.0000
+-netdev tap,fd=32,id=hostnet1,vhost=on,vhostfd=33 -device
+virtio-net-ccw,netdev=hostnet1,id=net1,mac=52:54:00:2a:e5:08,devno=fe.0.0002
+-chardev pty,id=charconsole0 -device
+sclpconsole,chardev=charconsole0,id=console0 -device
+virtio-balloon-ccw,id=balloon0,devno=fe.3.ffba -sandbox
+on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny
+-msg timestamp=on
+2018-07-17 15:48:42.252+0000: Domain id=21 is tainted: high-privileges
+2018-07-17T15:48:42.279380Z qemu-system-s390x: -chardev
+pty,id=charconsole0: char device redirected to /dev/pts/3 (label
+charconsole0)
+qemu-system-s390x: util/async.c:339: aio_get_linux_aio: Assertion
+`ctx->linux_aio' failed.
+2018-07-17 15:48:43.309+0000: shutting down, reason=failed
+
+
+Any help debugging this would be greatly appreciated.
+
+Thank you
+Farhan
+
+On 17.07.2018 [13:25:53 -0400], Farhan Ali wrote:
+>
+Hi,
+>
+>
+I am seeing some strange QEMU assertion failures for qemu on s390x,
+>
+which prevents a guest from starting.
+>
+>
+Git bisecting points to the following commit as the source of the error.
+>
+>
+commit ed6e2161715c527330f936d44af4c547f25f687e
+>
+Author: Nishanth Aravamudan <address@hidden>
+>
+Date:   Fri Jun 22 12:37:00 2018 -0700
+>
+>
+linux-aio: properly bubble up errors from initialization
+>
+>
+laio_init() can fail for a couple of reasons, which will lead to a NULL
+>
+pointer dereference in laio_attach_aio_context().
+>
+>
+To solve this, add a aio_setup_linux_aio() function which is called
+>
+early in raw_open_common. If this fails, propagate the error up. The
+>
+signature of aio_get_linux_aio() was not modified, because it seems
+>
+preferable to return the actual errno from the possible failing
+>
+initialization calls.
+>
+>
+Additionally, when the AioContext changes, we need to associate a
+>
+LinuxAioState with the new AioContext. Use the bdrv_attach_aio_context
+>
+callback and call the new aio_setup_linux_aio(), which will allocate a
+>
+new AioContext if needed, and return errors on failures. If it fails for
+>
+any reason, fallback to threaded AIO with an error message, as the
+>
+device is already in-use by the guest.
+>
+>
+Add an assert that aio_get_linux_aio() cannot return NULL.
+>
+>
+Signed-off-by: Nishanth Aravamudan <address@hidden>
+>
+Message-id: address@hidden
+>
+Signed-off-by: Stefan Hajnoczi <address@hidden>
+>
+>
+>
+Not sure what is causing this assertion to fail. Here is the qemu command
+>
+line of the guest, from qemu log, which throws this error:
+>
+>
+>
+LC_ALL=C PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin
+>
+QEMU_AUDIO_DRV=none /usr/local/bin/qemu-system-s390x -name
+>
+guest=rt_vm1,debug-threads=on -S -object
+>
+secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-21-rt_vm1/master-key.aes
+>
+-machine s390-ccw-virtio-2.12,accel=kvm,usb=off,dump-guest-core=off -m 1024
+>
+-realtime mlock=off -smp 4,sockets=4,cores=1,threads=1 -object
+>
+iothread,id=iothread1 -uuid 0cde16cd-091d-41bd-9ac2-5243df5c9a0d -display
+>
+none -no-user-config -nodefaults -chardev
+>
+socket,id=charmonitor,fd=28,server,nowait -mon
+>
+chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown -boot
+>
+strict=on -drive
+>
+file=/dev/mapper/360050763998b0883980000002a000031,format=raw,if=none,id=drive-virtio-disk0,cache=none,aio=native
+>
+-device
+>
+virtio-blk-ccw,iothread=iothread1,scsi=off,devno=fe.0.0001,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1,write-cache=on
+>
+-netdev tap,fd=30,id=hostnet0,vhost=on,vhostfd=31 -device
+>
+virtio-net-ccw,netdev=hostnet0,id=net0,mac=02:3a:c8:67:95:84,devno=fe.0.0000
+>
+-netdev tap,fd=32,id=hostnet1,vhost=on,vhostfd=33 -device
+>
+virtio-net-ccw,netdev=hostnet1,id=net1,mac=52:54:00:2a:e5:08,devno=fe.0.0002
+>
+-chardev pty,id=charconsole0 -device
+>
+sclpconsole,chardev=charconsole0,id=console0 -device
+>
+virtio-balloon-ccw,id=balloon0,devno=fe.3.ffba -sandbox
+>
+on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny -msg
+>
+timestamp=on
+>
+>
+>
+>
+2018-07-17 15:48:42.252+0000: Domain id=21 is tainted: high-privileges
+>
+2018-07-17T15:48:42.279380Z qemu-system-s390x: -chardev pty,id=charconsole0:
+>
+char device redirected to /dev/pts/3 (label charconsole0)
+>
+qemu-system-s390x: util/async.c:339: aio_get_linux_aio: Assertion
+>
+`ctx->linux_aio' failed.
+>
+2018-07-17 15:48:43.309+0000: shutting down, reason=failed
+>
+>
+>
+Any help debugging this would be greatly appreciated.
+iiuc, this possibly implies AIO was not actually used previously on this
+guest (it might have silently been falling back to threaded IO?). I
+don't have access to s390x, but would it be possible to run qemu under
+gdb and see if aio_setup_linux_aio is being called at all (I think it
+might not be, but I'm not sure why), and if so, if it's for the context
+in question?
+
+If it's not being called first, could you see what callpath is calling
+aio_get_linux_aio when this assertion trips?
+
+Thanks!
+-Nish
+
+On 07/17/2018 04:52 PM, Nishanth Aravamudan wrote:
+iiuc, this possibly implies AIO was not actually used previously on this
+guest (it might have silently been falling back to threaded IO?). I
+don't have access to s390x, but would it be possible to run qemu under
+gdb and see if aio_setup_linux_aio is being called at all (I think it
+might not be, but I'm not sure why), and if so, if it's for the context
+in question?
+
+If it's not being called first, could you see what callpath is calling
+aio_get_linux_aio when this assertion trips?
+
+Thanks!
+-Nish
+Hi Nishant,
+From the coredump of the guest this is the call trace that calls
+aio_get_linux_aio:
+Stack trace of thread 145158:
+#0  0x000003ff94dbe274 raise (libc.so.6)
+#1  0x000003ff94da39a8 abort (libc.so.6)
+#2  0x000003ff94db62ce __assert_fail_base (libc.so.6)
+#3  0x000003ff94db634c __assert_fail (libc.so.6)
+#4  0x000002aa20db067a aio_get_linux_aio (qemu-system-s390x)
+#5  0x000002aa20d229a8 raw_aio_plug (qemu-system-s390x)
+#6  0x000002aa20d309ee bdrv_io_plug (qemu-system-s390x)
+#7  0x000002aa20b5a8ea virtio_blk_handle_vq (qemu-system-s390x)
+#8  0x000002aa20db2f6e aio_dispatch_handlers (qemu-system-s390x)
+#9  0x000002aa20db3c34 aio_poll (qemu-system-s390x)
+#10 0x000002aa20be32a2 iothread_run (qemu-system-s390x)
+#11 0x000003ff94f879a8 start_thread (libpthread.so.0)
+#12 0x000003ff94e797ee thread_start (libc.so.6)
+
+
+Thanks for taking a look and responding.
+
+Thanks
+Farhan
+
+On 07/18/2018 09:42 AM, Farhan Ali wrote:
+On 07/17/2018 04:52 PM, Nishanth Aravamudan wrote:
+iiuc, this possibly implies AIO was not actually used previously on this
+guest (it might have silently been falling back to threaded IO?). I
+don't have access to s390x, but would it be possible to run qemu under
+gdb and see if aio_setup_linux_aio is being called at all (I think it
+might not be, but I'm not sure why), and if so, if it's for the context
+in question?
+
+If it's not being called first, could you see what callpath is calling
+aio_get_linux_aio when this assertion trips?
+
+Thanks!
+-Nish
+Hi Nishant,
+From the coredump of the guest this is the call trace that calls
+aio_get_linux_aio:
+Stack trace of thread 145158:
+#0  0x000003ff94dbe274 raise (libc.so.6)
+#1  0x000003ff94da39a8 abort (libc.so.6)
+#2  0x000003ff94db62ce __assert_fail_base (libc.so.6)
+#3  0x000003ff94db634c __assert_fail (libc.so.6)
+#4  0x000002aa20db067a aio_get_linux_aio (qemu-system-s390x)
+#5  0x000002aa20d229a8 raw_aio_plug (qemu-system-s390x)
+#6  0x000002aa20d309ee bdrv_io_plug (qemu-system-s390x)
+#7  0x000002aa20b5a8ea virtio_blk_handle_vq (qemu-system-s390x)
+#8  0x000002aa20db2f6e aio_dispatch_handlers (qemu-system-s390x)
+#9  0x000002aa20db3c34 aio_poll (qemu-system-s390x)
+#10 0x000002aa20be32a2 iothread_run (qemu-system-s390x)
+#11 0x000003ff94f879a8 start_thread (libpthread.so.0)
+#12 0x000003ff94e797ee thread_start (libc.so.6)
+
+
+Thanks for taking a look and responding.
+
+Thanks
+Farhan
+Trying to debug a little further, the block device in this case is a
+"host device". And looking at your commit carefully you use the
+bdrv_attach_aio_context callback to setup a Linux AioContext.
+For some reason the "host device" struct (BlockDriver bdrv_host_device
+in block/file-posix.c) does not have a bdrv_attach_aio_context defined.
+So a simple change of adding the callback to the struct solves the issue
+and the guest starts fine.
+diff --git a/block/file-posix.c b/block/file-posix.c
+index 28824aa..b8d59fb 100644
+--- a/block/file-posix.c
++++ b/block/file-posix.c
+@@ -3135,6 +3135,7 @@ static BlockDriver bdrv_host_device = {
+     .bdrv_refresh_limits = raw_refresh_limits,
+     .bdrv_io_plug = raw_aio_plug,
+     .bdrv_io_unplug = raw_aio_unplug,
++    .bdrv_attach_aio_context = raw_aio_attach_aio_context,
+
+     .bdrv_co_truncate       = raw_co_truncate,
+     .bdrv_getlength    = raw_getlength,
+I am not too familiar with block device code in QEMU, so not sure if
+this is the right fix or if there are some underlying problems.
+Thanks
+Farhan
+
+On 18.07.2018 [11:10:27 -0400], Farhan Ali wrote:
+>
+>
+>
+On 07/18/2018 09:42 AM, Farhan Ali wrote:
+>
+>
+>
+>
+>
+> On 07/17/2018 04:52 PM, Nishanth Aravamudan wrote:
+>
+> > iiuc, this possibly implies AIO was not actually used previously on this
+>
+> > guest (it might have silently been falling back to threaded IO?). I
+>
+> > don't have access to s390x, but would it be possible to run qemu under
+>
+> > gdb and see if aio_setup_linux_aio is being called at all (I think it
+>
+> > might not be, but I'm not sure why), and if so, if it's for the context
+>
+> > in question?
+>
+> >
+>
+> > If it's not being called first, could you see what callpath is calling
+>
+> > aio_get_linux_aio when this assertion trips?
+>
+> >
+>
+> > Thanks!
+>
+> > -Nish
+>
+>
+>
+>
+>
+> Hi Nishant,
+>
+>
+>
+>  From the coredump of the guest this is the call trace that calls
+>
+> aio_get_linux_aio:
+>
+>
+>
+>
+>
+> Stack trace of thread 145158:
+>
+> #0  0x000003ff94dbe274 raise (libc.so.6)
+>
+> #1  0x000003ff94da39a8 abort (libc.so.6)
+>
+> #2  0x000003ff94db62ce __assert_fail_base (libc.so.6)
+>
+> #3  0x000003ff94db634c __assert_fail (libc.so.6)
+>
+> #4  0x000002aa20db067a aio_get_linux_aio (qemu-system-s390x)
+>
+> #5  0x000002aa20d229a8 raw_aio_plug (qemu-system-s390x)
+>
+> #6  0x000002aa20d309ee bdrv_io_plug (qemu-system-s390x)
+>
+> #7  0x000002aa20b5a8ea virtio_blk_handle_vq (qemu-system-s390x)
+>
+> #8  0x000002aa20db2f6e aio_dispatch_handlers (qemu-system-s390x)
+>
+> #9  0x000002aa20db3c34 aio_poll (qemu-system-s390x)
+>
+> #10 0x000002aa20be32a2 iothread_run (qemu-system-s390x)
+>
+> #11 0x000003ff94f879a8 start_thread (libpthread.so.0)
+>
+> #12 0x000003ff94e797ee thread_start (libc.so.6)
+>
+>
+>
+>
+>
+> Thanks for taking a look and responding.
+>
+>
+>
+> Thanks
+>
+> Farhan
+>
+>
+>
+>
+>
+>
+>
+>
+Trying to debug a little further, the block device in this case is a "host
+>
+device". And looking at your commit carefully you use the
+>
+bdrv_attach_aio_context callback to setup a Linux AioContext.
+>
+>
+For some reason the "host device" struct (BlockDriver bdrv_host_device in
+>
+block/file-posix.c) does not have a bdrv_attach_aio_context defined.
+>
+So a simple change of adding the callback to the struct solves the issue and
+>
+the guest starts fine.
+>
+>
+>
+diff --git a/block/file-posix.c b/block/file-posix.c
+>
+index 28824aa..b8d59fb 100644
+>
+--- a/block/file-posix.c
+>
++++ b/block/file-posix.c
+>
+@@ -3135,6 +3135,7 @@ static BlockDriver bdrv_host_device = {
+>
+.bdrv_refresh_limits = raw_refresh_limits,
+>
+.bdrv_io_plug = raw_aio_plug,
+>
+.bdrv_io_unplug = raw_aio_unplug,
+>
++    .bdrv_attach_aio_context = raw_aio_attach_aio_context,
+>
+>
+.bdrv_co_truncate       = raw_co_truncate,
+>
+.bdrv_getlength    = raw_getlength,
+>
+>
+>
+>
+I am not too familiar with block device code in QEMU, so not sure if
+>
+this is the right fix or if there are some underlying problems.
+Oh this is quite embarassing! I only added the bdrv_attach_aio_context
+callback for the file-backed device. Your fix is definitely corect for
+host device. Let me make sure there weren't any others missed and I will
+send out a properly formatted patch. Thank you for the quick testing and
+turnaround!
+
+-Nish
+
+On 07/18/2018 08:52 PM, Nishanth Aravamudan wrote:
+>
+On 18.07.2018 [11:10:27 -0400], Farhan Ali wrote:
+>
+>
+>
+>
+>
+> On 07/18/2018 09:42 AM, Farhan Ali wrote:
+>
+>>
+>
+>>
+>
+>> On 07/17/2018 04:52 PM, Nishanth Aravamudan wrote:
+>
+>>> iiuc, this possibly implies AIO was not actually used previously on this
+>
+>>> guest (it might have silently been falling back to threaded IO?). I
+>
+>>> don't have access to s390x, but would it be possible to run qemu under
+>
+>>> gdb and see if aio_setup_linux_aio is being called at all (I think it
+>
+>>> might not be, but I'm not sure why), and if so, if it's for the context
+>
+>>> in question?
+>
+>>>
+>
+>>> If it's not being called first, could you see what callpath is calling
+>
+>>> aio_get_linux_aio when this assertion trips?
+>
+>>>
+>
+>>> Thanks!
+>
+>>> -Nish
+>
+>>
+>
+>>
+>
+>> Hi Nishant,
+>
+>>
+>
+>>  From the coredump of the guest this is the call trace that calls
+>
+>> aio_get_linux_aio:
+>
+>>
+>
+>>
+>
+>> Stack trace of thread 145158:
+>
+>> #0  0x000003ff94dbe274 raise (libc.so.6)
+>
+>> #1  0x000003ff94da39a8 abort (libc.so.6)
+>
+>> #2  0x000003ff94db62ce __assert_fail_base (libc.so.6)
+>
+>> #3  0x000003ff94db634c __assert_fail (libc.so.6)
+>
+>> #4  0x000002aa20db067a aio_get_linux_aio (qemu-system-s390x)
+>
+>> #5  0x000002aa20d229a8 raw_aio_plug (qemu-system-s390x)
+>
+>> #6  0x000002aa20d309ee bdrv_io_plug (qemu-system-s390x)
+>
+>> #7  0x000002aa20b5a8ea virtio_blk_handle_vq (qemu-system-s390x)
+>
+>> #8  0x000002aa20db2f6e aio_dispatch_handlers (qemu-system-s390x)
+>
+>> #9  0x000002aa20db3c34 aio_poll (qemu-system-s390x)
+>
+>> #10 0x000002aa20be32a2 iothread_run (qemu-system-s390x)
+>
+>> #11 0x000003ff94f879a8 start_thread (libpthread.so.0)
+>
+>> #12 0x000003ff94e797ee thread_start (libc.so.6)
+>
+>>
+>
+>>
+>
+>> Thanks for taking a look and responding.
+>
+>>
+>
+>> Thanks
+>
+>> Farhan
+>
+>>
+>
+>>
+>
+>>
+>
+>
+>
+> Trying to debug a little further, the block device in this case is a "host
+>
+> device". And looking at your commit carefully you use the
+>
+> bdrv_attach_aio_context callback to setup a Linux AioContext.
+>
+>
+>
+> For some reason the "host device" struct (BlockDriver bdrv_host_device in
+>
+> block/file-posix.c) does not have a bdrv_attach_aio_context defined.
+>
+> So a simple change of adding the callback to the struct solves the issue and
+>
+> the guest starts fine.
+>
+>
+>
+>
+>
+> diff --git a/block/file-posix.c b/block/file-posix.c
+>
+> index 28824aa..b8d59fb 100644
+>
+> --- a/block/file-posix.c
+>
+> +++ b/block/file-posix.c
+>
+> @@ -3135,6 +3135,7 @@ static BlockDriver bdrv_host_device = {
+>
+>      .bdrv_refresh_limits = raw_refresh_limits,
+>
+>      .bdrv_io_plug = raw_aio_plug,
+>
+>      .bdrv_io_unplug = raw_aio_unplug,
+>
+> +    .bdrv_attach_aio_context = raw_aio_attach_aio_context,
+>
+>
+>
+>      .bdrv_co_truncate       = raw_co_truncate,
+>
+>      .bdrv_getlength    = raw_getlength,
+>
+>
+>
+>
+>
+>
+>
+> I am not too familiar with block device code in QEMU, so not sure if
+>
+> this is the right fix or if there are some underlying problems.
+>
+>
+Oh this is quite embarassing! I only added the bdrv_attach_aio_context
+>
+callback for the file-backed device. Your fix is definitely corect for
+>
+host device. Let me make sure there weren't any others missed and I will
+>
+send out a properly formatted patch. Thank you for the quick testing and
+>
+turnaround!
+Farhan, can you respin your patch with proper sign-off and patch description?
+Adding qemu-block.
+
+Hi Christian,
+
+On 19.07.2018 [08:55:20 +0200], Christian Borntraeger wrote:
+>
+>
+>
+On 07/18/2018 08:52 PM, Nishanth Aravamudan wrote:
+>
+> On 18.07.2018 [11:10:27 -0400], Farhan Ali wrote:
+>
+>>
+>
+>>
+>
+>> On 07/18/2018 09:42 AM, Farhan Ali wrote:
+<snip>
+
+>
+>> I am not too familiar with block device code in QEMU, so not sure if
+>
+>> this is the right fix or if there are some underlying problems.
+>
+>
+>
+> Oh this is quite embarassing! I only added the bdrv_attach_aio_context
+>
+> callback for the file-backed device. Your fix is definitely corect for
+>
+> host device. Let me make sure there weren't any others missed and I will
+>
+> send out a properly formatted patch. Thank you for the quick testing and
+>
+> turnaround!
+>
+>
+Farhan, can you respin your patch with proper sign-off and patch description?
+>
+Adding qemu-block.
+I sent it yesterday, sorry I didn't cc everyone from this e-mail:
+http://lists.nongnu.org/archive/html/qemu-block/2018-07/msg00516.html
+Thanks,
+Nish
+