linux-kernel - Re: [PATCH] drm/virtio: fix mmap page attributes

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Date:   Tue, 10 Dec 2019 10:27:14 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Gerd Hoffmann <kraxel@...hat.com>, dri-devel@...ts.freedesktop.org
Cc:     David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@...ts.linux-foundation.org>,
        gurchetansingh@...omium.org
Subject: Re: [PATCH] drm/virtio: fix mmap page attributes

Hi

Am 10.12.19 um 09:57 schrieb Gerd Hoffmann:
> virtio-gpu uses cached mappings.  shmem helpers use writecombine though.
> So roll our own mmap function, wrapping drm_gem_shmem_mmap(), to tweak
> vm_page_prot accordingly.
> 
> Reported-by: Gurchetan Singh <gurchetansingh@...omium.org>
> Signed-off-by: Gerd Hoffmann <kraxel@...hat.com>
> ---
>  drivers/gpu/drm/virtio/virtgpu_object.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/virtio/virtgpu_object.c b/drivers/gpu/drm/virtio/virtgpu_object.c
> index 017a9e0fc3bb..158610902054 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_object.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_object.c
> @@ -75,6 +75,22 @@ static void virtio_gpu_free_object(struct drm_gem_object *obj)
>  	drm_gem_shmem_free_object(obj);
>  }
>  
> +static int virtio_gpu_gem_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
> +{
> +	pgprot_t prot;
> +	int ret;
> +
> +	ret = drm_gem_shmem_mmap(obj, vma);
> +	if (ret < 0)
> +		return ret;
> +
> +	/* virtio-gpu needs normal caching, so clear writecombine */
> +	prot = vm_get_page_prot(vma->vm_flags);
> +	prot = pgprot_decrypted(prot);
> +	vma->vm_page_prot = prot;
> +	return 0;
> +}

There's similar code in udl, [1] which still uses writecombine for
imported buffers. Virtio does not need this?

Aside from this, do you think we could handle all special cases within
shmem?

Best regards
Thomas

[1]
https://cgit.freedesktop.org/drm/drm-tip/tree/drivers/gpu/drm/udl/udl_gem.c?id=28ecf94a6f1072fc4744c06f5b3d267297125b37#n20

> +
>  static const struct drm_gem_object_funcs virtio_gpu_gem_funcs = {
>  	.free = virtio_gpu_free_object,
>  	.open = virtio_gpu_gem_object_open,
> @@ -86,7 +102,7 @@ static const struct drm_gem_object_funcs virtio_gpu_gem_funcs = {
>  	.get_sg_table = drm_gem_shmem_get_sg_table,
>  	.vmap = drm_gem_shmem_vmap,
>  	.vunmap = drm_gem_shmem_vunmap,
> -	.mmap = &drm_gem_shmem_mmap,
> +	.mmap = &virtio_gpu_gem_mmap,
>  };
>  
>  struct drm_gem_object *virtio_gpu_create_object(struct drm_device *dev,
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer



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