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

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Message-ID: <2f51584c-0590-432b-a4db-7a2af99cde15@collabora.com>
Date: Sun, 19 Jan 2025 14:50:07 +0300
From: Dmitry Osipenko <dmitry.osipenko@...labora.com>
To: fnkl.kernel@...il.com, 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>
Cc: 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

Hi,

On 1/10/25 00:45, Sasha Finkelstein via B4 Relay wrote:
> From: Sasha Finkelstein <fnkl.kernel@...il.com>
> 
> Those are useful to implement coherent cross-vm mmap.
> 
> Signed-off-by: Sasha Finkelstein <fnkl.kernel@...il.com>
> ---
>  drivers/gpu/drm/virtio/virtgpu_vram.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/virtio/virtgpu_vram.c b/drivers/gpu/drm/virtio/virtgpu_vram.c
> index 25df81c027837c248a746e41856b5aa7e216b8d5..64e2c6dbdd678ac4c0da89fdd4c9dbf937c2c335 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_vram.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_vram.c
> @@ -56,12 +56,11 @@ static int virtio_gpu_vram_mmap(struct drm_gem_object *obj,
>  	else if (vram->map_info == VIRTIO_GPU_MAP_CACHE_UNCACHED)
>  		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>  
> -	/* Partial mappings of GEM buffers don't happen much in practice. */
> -	if (vm_size != vram->vram_node.size)
> +	if (vm_size > vram->vram_node.size)
>  		return -EINVAL;
>  
>  	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. The
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().

BTW, partial mapping is wanted by Intel native context that I work on.
The intent of this patch is correct. There are practical uses for
partial mappings.

Please correct vma->vm_pgoff in v2.

-- 
Best regards,
Dmitry