Re: [QEMU PATCH v4 10/13] virtio-gpu: Resource UUID

[Akihiko Odaki]

On 2023/09/13 20:34, Albert Esteve wrote:
> On Wed, Sep 13, 2023 at 12:34 PM Akihiko Odaki <akihiko.odaki@daynix.com 
> <mailto:akihiko.odaki@daynix.com>> wrote:
>
>     On 2023/09/13 16:55, Albert Esteve wrote:
>      > Hi Antonio,
>      >
>      > If I'm not mistaken, this patch is related with:
>      >
>     https://lists.gnu.org/archive/html/qemu-devel/2023-09/msg01853.html
>     <https://lists.gnu.org/archive/html/qemu-devel/2023-09/msg01853.html>
>      >
>     <https://lists.gnu.org/archive/html/qemu-devel/2023-09/msg01853.html
>     <https://lists.gnu.org/archive/html/qemu-devel/2023-09/msg01853.html>>
>      > IMHO, ideally, virtio-gpu and vhost-user-gpu both, would use the
>      > infrastructure from the patch I linked to store the
>      > virtio objects, so that they can be later shared with other devices.
>
>     I don't think such sharing is possible because the resources are
>     identified by IDs that are local to the device. That also complicates
>     migration.
>
>     Regards,
>     Akihiko Odaki
>
> Hi Akihiko,
>
> As far as I understand, the feature to export dma-bufs from the
> virtgpu was introduced as part of the virtio cross-device sharing
> proposal [1]. Thus, it shall be posible. When virtgpu ASSING_UUID,
> it exports and identifies the dmabuf resource, so that when the dmabuf gets
> shared inside the guest (e.g., with virtio-video), we can use the assigned
> UUID to find the dmabuf in the host (using the patch that I linked above),
> and import it.
>
> [1] - https://lwn.net/Articles/828988/ <https://lwn.net/Articles/828988/>

The problem is that virtio-gpu can have other kind of resources like 
pixman and OpenGL textures and manage them and DMA-BUFs with unified 
resource ID.

So you cannot change:
g_hash_table_insert(g->resource_uuids, 
GUINT_TO_POINTER(assign.resource_id), uuid);
by:
virtio_add_dmabuf(uuid, assign.resource_id);

assign.resource_id is not DMA-BUF file descriptor, and the underlying 
resource my not be DMA-BUF at first place.

Also, since this lives in the common code that is not used only by 
virtio-gpu-gl but also virtio-gpu, which supports migration, we also 
need to take care of that. It is not a problem for DMA-BUF as DMA-BUF is 
not migratable anyway, but the situation is different in this case.

Implementing cross-device sharing is certainly a possibility, but that 
requires more than dealing with DMA-BUFs.