Re: [PATCH v2] hw/display/virtio-gpu: Reject non-migratable large blobs

Hi

On Tue, Jul 28, 2026 at 10:26 AM Akihiko Odaki
<[email protected]> wrote:
>
> On 2026/07/27 18:30, Daniel P. Berrangé wrote:
> > On Mon, Jul 27, 2026 at 06:23:25PM +0900, Akihiko Odaki wrote:
> >> The blob size is encoded as a 64-bit integer in the virtio protocol, but
> >> it is encoded as a 32-bit integer in the migration stream, and
> >> a large blob whose size cannot be expressed in a 32-bit integer will
> >> be corrupted after migration. Deny creating such large blobs.
> >
> > Is there a valid use case for such large blobs ?   If so, rather than
> > rejecting the blob, we could dynamically add a migration  blocker.
> > That would allow the valid use cases, while preventing the migration
> > problem.
>
> For the QEMU code path changed here, there should be no practical use
> case for a blob larger than 4 GiB. The non-virgl implementation uses
> blobs as scanout or cursor backing. Such a size is excessive for those uses.
>
> A dynamic migration blocker would also let the guest make an otherwise
> migratable VM non-migratable at runtime merely by creating such a
> resource. This could interfere with host-level VM management. So I think
> rejecting sizes that the current migration format cannot represent is
> better for now.

What's the difference? The dynamic blocker would simply happen earlier
or be queryable, no?

> If a concrete use case emerges, extending the migration format would be
> preferable to making migratability depend on guest-controlled state.
>
> Regards,
> Akihiko Odaki
>
> >
> >>
> >> Fixes: f66767f75c9c ("virtio-gpu: add virtio-gpu/blob vmstate subsection")
> >> Fixes: 10b9ddbc83b9 ("Revert "virtio-gpu: block migration of VMs with blob=true"")
> >> Signed-off-by: Akihiko Odaki <[email protected]>
> >> ---
> >> Changes in v2:
> >> - Add the Fixes: tag for f66767f75c9c.
> >> - Add a TODO to encode blob_size as 64 bits in virtio-gpu/blob
> >>    VMState version 2.
> >> - Link to v1: https://lore.kernel.org/qemu-devel/[email protected]
> >> ---
> >>   hw/display/virtio-gpu.c | 15 +++++++++++++++
> >>   1 file changed, 15 insertions(+)
> >>
> >> diff --git a/hw/display/virtio-gpu.c b/hw/display/virtio-gpu.c
> >> index 718ba3039290..987b1b3ea860 100644
> >> --- a/hw/display/virtio-gpu.c
> >> +++ b/hw/display/virtio-gpu.c
> >> @@ -352,6 +352,14 @@ static void virtio_gpu_resource_create_blob(VirtIOGPU *g,
> >>           return;
> >>       }
> >>
> >> +    if (cblob.size > UINT32_MAX) {
> >> +        qemu_log_mask(LOG_GUEST_ERROR,
> >> +                      "%s: blob too large (%" PRIu64 "> %" PRIu32 ")\n",
> >> +                      __func__, cblob.size, UINT32_MAX);
> >> +        cmd->error = VIRTIO_GPU_RESP_ERR_OUT_OF_MEMORY;
> >> +        return;
> >> +    }
> >> +
> >>       if (virtio_gpu_find_resource(g, cblob.resource_id)) {
> >>           qemu_log_mask(LOG_GUEST_ERROR, "%s: resource already exists %d\n",
> >>                         __func__, cblob.resource_id);
> >> @@ -1435,7 +1443,14 @@ static int virtio_gpu_blob_save(QEMUFile *f, void *opaque, size_t size,
> >>           }
> >>           assert(!res->image);
> >>           qemu_put_be32(f, res->resource_id);
> >> +
> >> +        /*
> >> +         * The migration stream encodes blob_size as a 32-bit integer for
> >> +         * compatibility though virtio encodes it as a 64-bit integer. A newer
> >> +         * version of the migration stream should encode it as a 64-bit integer.
> >> +         */
> >>           qemu_put_be32(f, res->blob_size);
> >> +
> >>           qemu_put_be32(f, res->iov_cnt);
> >>           for (i = 0; i < res->iov_cnt; i++) {
> >>               qemu_put_be64(f, res->addrs[i]);
> >>
> >> ---
> >> base-commit: 006a22cb26998998385b104db1ff9466ef2f3153
> >> change-id: 20260725-blob-8a6a1a85601a
> >>
> >> Best regards,
> >> --
> >> Akihiko Odaki <[email protected]>
> >>
> >>
> >
> > With regards,
> > Daniel
>
>


-- 
Marc-André Lureau