s390x: virtio-mem support

Let's add our virtio-mem-ccw proxy device and wire it up. We should
be supporting everything (e.g., device unplug, "dynamic-memslots") that
we already support for the virtio-pci variant.

With a Linux guest that supports virtio-mem (and has automatic memory
onlining properly configured) the following example will work:

1. Start a VM with 4G initial memory and a virtio-mem device with a maximum
   capacity of 16GB:

   qemu/build/qemu-system-s390x \
    --enable-kvm \
    -m 4G,maxmem=20G \
    -nographic \
    -smp 8 \
    -hda Fedora-Server-KVM-40-1.14.s390x.qcow2 \
    -chardev socket,id=monitor,path=/var/tmp/monitor,server,nowait \
    -mon chardev=monitor,mode=readline \
    -object memory-backend-ram,id=mem0,size=16G,reserve=off \
    -device virtio-mem-ccw,id=vmem0,memdev=mem0,dynamic-memslots=on

2. Query the current size of virtio-mem device:

    (qemu) info memory-devices
    Memory device [virtio-mem]: "vmem0"
      memaddr: 0x100000000
      node: 0
      requested-size: 0
      size: 0
      max-size: 17179869184
      block-size: 1048576
      memdev: /objects/mem0

3. Request to grow it to 8GB (hotplug 8GB):

    (qemu) qom-set vmem0 requested-size 8G
    (qemu) info memory-devices
    Memory device [virtio-mem]: "vmem0"
      memaddr: 0x100000000
      node: 0
      requested-size: 8589934592
      size: 8589934592
      max-size: 17179869184
      block-size: 1048576
      memdev: /objects/mem0

4. Request to grow to 16GB (hotplug another 8GB):

    (qemu) qom-set vmem0 requested-size 16G
    (qemu) info memory-devices
    Memory device [virtio-mem]: "vmem0"
      memaddr: 0x100000000
      node: 0
      requested-size: 17179869184
      size: 17179869184
      max-size: 17179869184
      block-size: 1048576
      memdev: /objects/mem0

5. Try to hotunplug all memory again, shrinking to 0GB:

    (qemu) qom-set vmem0 requested-size 0G
    (qemu) info memory-devices
    Memory device [virtio-mem]: "vmem0"
      memaddr: 0x100000000
      node: 0
      requested-size: 0
      size: 0
      max-size: 17179869184
      block-size: 1048576
      memdev: /objects/mem0

6. If it worked, unplug the device

    (qemu) device_del vmem0
    (qemu) info memory-devices
    (qemu) object_del mem0

7. Hotplug a new device with a smaller capacity and directly size it to 1GB

    (qemu) object_add memory-backend-ram,id=mem0,size=8G,reserve=off
    (qemu) device_add virtio-mem-ccw,id=vmem0,memdev=mem0,\
                      dynamic-memslots=on,requested-size=1G
    (qemu) info memory-devices
    Memory device [virtio-mem]: "vmem0"
      memaddr: 0x100000000
      node: 0
      requested-size: 1073741824
      size: 1073741824
      max-size: 8589934592
      block-size: 1048576
      memdev: /objects/mem0

Trying to use a virtio-mem device backed by hugetlb into a !hugetlb VM
correctly results in the error:
   ... Memory device uses a bigger page size than initial memory

Note that the virtio-mem driver in Linux will supports 1 MiB (pageblock)
granularity.

Message-ID: <20241219144115.2820241-15-david@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
index a0dceaddec..48e4a58239 100644
--- a/hw/virtio/virtio-mem.c
+++ b/hw/virtio/virtio-mem.c
@@ -61,6 +61,8 @@ static uint32_t virtio_mem_default_thp_size(void)
     } else if (qemu_real_host_page_size() == 64 * KiB) {
         default_thp_size = 512 * MiB;
     }
+#elif defined(__s390x__)
+    default_thp_size = 1 * MiB;
 #endif
 
     return default_thp_size;
@@ -168,7 +170,7 @@ static bool virtio_mem_has_shared_zeropage(RAMBlock *rb)
  * necessary (as the section size can change). But it's more likely that the
  * section size will rather get smaller and not bigger over time.
  */
-#if defined(TARGET_X86_64) || defined(TARGET_I386)
+#if defined(TARGET_X86_64) || defined(TARGET_I386) || defined(TARGET_S390X)
 #define VIRTIO_MEM_USABLE_EXTENT (2 * (128 * MiB))
 #elif defined(TARGET_ARM)
 #define VIRTIO_MEM_USABLE_EXTENT (2 * (512 * MiB))