Re: [PATCH] drm/virtio: Support partial maps of GEM objects

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

On 1/19/25 19:18, Sasha Finkelstein wrote:
> On Sun, 19 Jan 2025 at 12:50, Dmitry Osipenko
> <dmitry.osipenko@collabora.com> wrote:
>>>       ret = io_remap_pfn_range(vma, vma->vm_start,
>>> -                              vram->vram_node.start >> PAGE_SHIFT,
>>> +                              (vram->vram_node.start >> PAGE_SHIFT) + vma->vm_pgoff,
>>>                                vm_size, vma->vm_page_prot);
>>>       return ret;
>>>  }
>>
>> The vma->vm_pgoff is fake in DRM, it's used for looking up DRM GEM
>> object based on the vma->vm_pgoff value when mmap is invoked.
> 
> If my understanding is correct, vm_pgoff gets "unfaked" by
> https://elixir.bootlin.com/linux/v6.12.6/source/drivers/gpu/drm/virtio/virtgpu_vram.c#L48
> 
>> vma->vm_pgoff should be treated as zero here. Hence we can map a part of
>> GEM, but only from its start. See drm_gem_mmap().
> 
> I've had a "v0" (not on ml) of this patch that always treated vma->vm_pgoff as
> zero. This broke when anything tried to mmap with a non-zero offset. Adding
> vm_pgoff made it work correctly.

I've tested this patch. Partial mapping with a non-zero offset doesn't
work because drm_gem_mmap() rejects it. I'd want to see your sample code
that performs mmaping, maybe I'm missing something.

>> Please correct vma->vm_pgoff in v2.
> 
> I need apps to be able to mmap with a non-zero offset for my usecase.
> While the correct value may be something else other than what is in
> the current patch, 0 is definitely incorrect for at least some workloads.

drm_gem_mmap() uses drm_vma_offset_exact_lookup_locked() that doesn't
allow vma->vm_pgoff != node.start. AFAICT, no one driver supports
mapping with a non-zero offset, perhaps for a reason that I don't know
about.

-- 
Best regards,
Dmitry