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

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Message-ID: <a6b7a3a1-e6f9-4da7-96b3-62aefb82dced@collabora.com>
Date: Sun, 19 Jan 2025 23:02:34 +0300
From: Dmitry Osipenko <dmitry.osipenko@...labora.com>
To: Sasha Finkelstein <fnkl.kernel@...il.com>
Cc: David Airlie <airlied@...hat.com>, Gerd Hoffmann <kraxel@...hat.com>,
 Gurchetan Singh <gurchetansingh@...omium.org>, Chia-I Wu
 <olvaffe@...il.com>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 Simona Vetter <simona@...ll.ch>, dri-devel@...ts.freedesktop.org,
 virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org,
 asahi@...ts.linux.dev
Subject: Re: [PATCH] drm/virtio: Support partial maps of GEM objects

On 1/19/25 19:18, Sasha Finkelstein wrote:
> On Sun, 19 Jan 2025 at 12:50, Dmitry Osipenko
> <dmitry.osipenko@...labora.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