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

[Sergio Lopez Pascual]

Gurchetan Singh <gurchetansingh at chromium.org> writes:

> 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?

This is true, but finding a common place to put the page size is really
hard in practice. I don't think we can borrow space in the feature bits
for that (and that would probably be abusing its purpose quite a bit)
and extending the transport configuration registers is quite cumbersome
and, in general, undesirable.

That leaves us with the device-specific config space, and that implies a
device-specific feature bit as it's implemented in this series.

The Shared Memory Regions on the VIRTIO spec, while doesn't talk
specifically about page size, also gives us a hint about this being the
right direction:

"
2.10 Shared Memory Regions
(...)
Memory consistency rules vary depending on the region and the device
and they will be specified as required by each device."
"

Thanks,
Sergio.