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

[Sasha Finkelstein]

On Sun, 19 Jan 2025 at 12:50, Dmitry Osipenko
<dmitry.osipenko at 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.

> 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.