[PATCH 0/2] drm/virtio: introduce the HOST_PAGE_SIZE feature

[Gurchetan Singh]

On Tue, Aug 6, 2024 at 1:15 PM Rob Clark <robdclark at gmail.com> wrote:

> On Tue, Aug 6, 2024 at 9:15 AM Gurchetan Singh
> <gurchetansingh at chromium.org> wrote:
> >
> >
> >
> > On Mon, Aug 5, 2024 at 2:14 AM Sergio Lopez Pascual <slp at redhat.com>
> wrote:
> >>
> >> Dmitry Osipenko <dmitry.osipenko at collabora.com> writes:
> >>
> >> > On 7/23/24 14:49, Sergio Lopez wrote:
> >> >> There's an incresing number of machines supporting multiple page
> sizes
> >> >> and on these machines the host and a guest can be running, each one,
> >> >> with a different page size.
> >> >>
> >> >> For what pertains to virtio-gpu, this is not a problem if the page
> size
> >> >> of the guest happens to be bigger or equal than the host, but will
> >> >> potentially lead to failures in memory allocations and/or mappings
> >> >> otherwise.
> >> >
> >> > Please describe concrete problem you're trying to solve. Guest memory
> >> > allocation consists of guest pages, I don't see how knowledge of host
> >> > page size helps anything in userspace.
> >> >
> >> > I suspect you want this for host blobs, but then it should be
> >> > virtio_gpu_vram_create() that should use max(host_page_sz,
> >> > guest_page_size), AFAICT. It's kernel who is responsible for memory
> >> > management, userspace can't be trusted for doing that.
> >>
> >> Mesa's Vulkan/Venus uses CREATE_BLOB to request the host the creation
> >> and mapping into the guest of device-backed memory and shmem regions.
> >> The CREATE_BLOB ioctl doesn't update drm_virtgpu_resource_create->size,
> >> so the guest kernel (and, as a consequence, the host kernel) can't
> >> override the user's request.
> >>
> >> I'd like Mesa's Vulkan/Venus in the guest to be able to obtain the host
> >> page size to align the size of the CREATE_BLOB requests as required.
> >
> >
> > gfxstream solves this problem by putting the relevant information in the
> capabilities obtained from the host:
> >
> >
> https://android.googlesource.com/platform/hardware/google/gfxstream/+/refs/heads/main/host/virtio-gpu-gfxstream-renderer.cpp#1691
> >
> > If you want to be paranoid, you can also validate the
> ResourceCreateBlob::size is properly host-page aligned when that request
> reaches the host.
> >
> > So you can probably solve this problem using current interfaces.
> Whether it's cleaner for all context types to use the capabilities, or have
> all VMMs to expose VIRTIO_GPU_F_HOST_PAGE_SIZE, would be the cost/benefit
> tradeoff.
> >
>
> I guess solving it in a context-type specific way is possible.  But I
> think it is a relatively universal constraint.  And maybe it makes
> sense for virtgpu guest kernel to enforce alignment (at least it can
> return an error synchronously) in addition to the host.
>

virtio-media may have support for VIRTIO_MEDIA_CMD_MMAP too, so could run
into this issue.

https://github.com/chromeos/virtio-media?tab=readme-ov-file#shared-memory-regions

virtio-fs also has the DAX window which uses the same memory mapping
mechanism.

https://virtio-fs.gitlab.io/design.html

Maybe this should not be a virtio-gpu thing, but a virtio thing?


>
> BR,
> -R
>
> >>
> >>
> >> Thanks,
> >> Sergio.
> >>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/dri-devel/attachments/20240807/f0d52db1/attachment.htm>