linux-kernel - Re: [PATCH 0/2] drm/virtio: introduce the HOST_PAGE

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Message-ID: <CAF6AEGsnpEYFsCfZUAPopWzY=wv_GWn0P5f5D6U9y-JrWGQVnw@mail.gmail.com>
Date: Tue, 6 Aug 2024 13:15:14 -0700
From: Rob Clark <robdclark@...il.com>
To: Gurchetan Singh <gurchetansingh@...omium.org>
Cc: Sergio Lopez Pascual <slp@...hat.com>, Dmitry Osipenko <dmitry.osipenko@...labora.com>, tzimmermann@...e.de, 
	mripard@...nel.org, olvaffe@...il.com, kraxel@...hat.com, daniel@...ll.ch, 
	maarten.lankhorst@...ux.intel.com, airlied@...hat.com, 
	linux-kernel@...r.kernel.org, virtualization@...ts.linux.dev, 
	dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 0/2] drm/virtio: introduce the HOST_PAGE_SIZE feature

On Tue, Aug 6, 2024 at 9:15 AM Gurchetan Singh
<gurchetansingh@...omium.org> wrote:
>
>
>
> On Mon, Aug 5, 2024 at 2:14 AM Sergio Lopez Pascual <slp@...hat.com> wrote:
>>
>> Dmitry Osipenko <dmitry.osipenko@...labora.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.

BR,
-R

>>
>>
>> Thanks,
>> Sergio.
>>