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

["Daniel P. Berrangé" <berrange@redhat.com>]

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.

> 
> 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 <odaki@rsg.ci.i.u-tokyo.ac.jp>
> ---
> 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/20260725-blob-v1-1-f643fbd76398@rsg.ci.i.u-tokyo.ac.jp
> ---
>  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 <odaki@rsg.ci.i.u-tokyo.ac.jp>
> 
> 

With regards,
Daniel
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