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

[Sergio Lopez Pascual]

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.

Thanks,
Sergio.