Re: [PULL 5/5] virtio-gpu: fix scanout migration post-load

[Fiona Ebner <f.ebner@proxmox.com>]

Am 12.03.24 um 15:02 schrieb marcandre.lureau@redhat.com:
> From: Marc-André Lureau <marcandre.lureau@redhat.com>
> 
> The current post-loading code for scanout has a FIXME: it doesn't take
> the resource region/rect into account. But there is more, when adding
> blob migration support in commit f66767f75c9, I didn't realize that blob
> resources could be used for scanouts. This situationn leads to a crash
> during post-load, as they don't have an associated res->image.
> 
> virtio_gpu_do_set_scanout() handle all cases, but requires the
> associated virtio_gpu_framebuffer, which is currently not saved during
> migration.
> 
> Add a v2 of "virtio-gpu-one-scanout" with the framebuffer fields, so we
> can restore blob scanouts, as well as fixing the existing FIXME.
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> Reviewed-by: Sebastian Ott <sebott@redhat.com>

Hi,
unfortunately, this broke migration from pre-9.0 to 9.0:

> vmstate_load_state_field virtio-gpu:virtio-gpu
> vmstate_load_state_field virtio-gpu-scanouts:parent_obj.enable
> vmstate_load_state_field virtio-gpu-scanouts:parent_obj.conf.max_outputs
> vmstate_load_state_field virtio-gpu-scanouts:parent_obj.scanout
> vmstate_load_state_field virtio-gpu-one-scanout:resource_id
> vmstate_load_state_field virtio-gpu-one-scanout:width
> vmstate_load_state_field virtio-gpu-one-scanout:height
> vmstate_load_state_field virtio-gpu-one-scanout:x
> vmstate_load_state_field virtio-gpu-one-scanout:y
> vmstate_load_state_field virtio-gpu-one-scanout:cursor.resource_id
> vmstate_load_state_field virtio-gpu-one-scanout:cursor.hot_x
> vmstate_load_state_field virtio-gpu-one-scanout:cursor.hot_y
> vmstate_load_state_field virtio-gpu-one-scanout:cursor.pos.x
> vmstate_load_state_field virtio-gpu-one-scanout:cursor.pos.y
> vmstate_load_state_field virtio-gpu-one-scanout:fb.format
> vmstate_load_state_field virtio-gpu-one-scanout:fb.bytes_pp
> vmstate_load_state_field virtio-gpu-one-scanout:fb.width
> vmstate_load_state_field virtio-gpu-one-scanout:fb.height
> vmstate_load_state_field virtio-gpu-one-scanout:fb.stride
> vmstate_load_state_field virtio-gpu-one-scanout:fb.offset
> qemu-system-x86_64: Missing section footer for 0000:00:02.0/virtio-gpu
> qemu-system-x86_64: Error -22 while loading VM state

It wrongly tries to load the fb fields even though they should be
guarded by version 2.

Looking at it with GDB, in vmstate_load_state(), when we come to
field->name == parent_obj.scanout, the

>                 } else if (field->flags & VMS_STRUCT) {
>                     ret = vmstate_load_state(f, field->vmsd, curr_elem,
>                                              field->vmsd->version_id);

branch will be taken and suddenly we'll have a call to
vmstate_load_state() for vmsd==vmstate_virtio_gpu_scanout with
version_id==2 rather than version_id==1, because that is
field->vmsd->version_id (i.e. the .version_id in VMStateDescription
vmstate_virtio_gpu_scanout).

Would it have been necessary to version the VMStateDescription
vmstate_virtio_gpu_scanouts too using VMS_VSTRUCT (or am I
misinterpreting the use case for that)?

Best Regards,
Fiona