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

[Albert Esteve <aesteve@xxxxxxxxxx>]

On Wed, Sep 13, 2023 at 2:22 PM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx> wrote:

On 2023/09/13 20:34, Albert Esteve wrote:
>
>
> On Wed, Sep 13, 2023 at 12:34 PM Akihiko Odaki <akihiko.odaki@xxxxxxxxxx
> <mailto:akihiko.odaki@xxxxxxxxxx>> 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.

I see.

 

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.

I didn't really look into the patch in-depth, so the code was intended

to give an idea of how the implementation would look like with

the cross-device patch API. Indeed, it is not the resource_id,

(I just took a brief look at the virtio specificacion 1.2), but the underlying

resource what we want to use here.

 

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.

So, if I understood correctly, dmabufs are just a subset of the resources

that the gpu manages, or can assign UUIDs to. I am not sure why

the virt gpu driver would want to send a ASSIGN_UUID for anything

that is not a dmabuf (are we sure it does?), but I am not super familiarized

with virtgpu to begin with.

But I see that internally, the GPU specs relate a UUID with a resource_id,

so we still need both tables:

- one to relate UUID with resource_id to be able to locate the underlying resource

- the table that holds the dmabuf with the UUID for cross-device sharing

With that in mind, sounds to me that the support for cross-device sharing could

be added on top of this patch, once https://lists.gnu.org/archive/html/qemu-devel/2023-09/msg01850.html

lands.

I hope that makes sense.

Regards,

Albert