linux-kernel - Re: [PATCH 4/4] drm/virtio: Add memory barriers for capset cache.

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Date:   Thu, 6 Jun 2019 09:41:22 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     davidriley@...omium.org
Cc:     dri-devel@...ts.freedesktop.org,
        virtualization@...ts.linux-foundation.org,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] drm/virtio: Add memory barriers for capset cache.

On Wed, Jun 05, 2019 at 04:44:23PM -0700, davidriley@...omium.org wrote:
> From: David Riley <davidriley@...omium.org>
> 
> After data is copied to the cache entry, atomic_set is used indicate
> that the data is the entry is valid without appropriate memory barriers.
> Similarly the read side was missing the same memory barries.
> 
> Signed-off-by: David Riley <davidriley@...omium.org>
> ---
>  drivers/gpu/drm/virtio/virtgpu_ioctl.c | 3 +++
>  drivers/gpu/drm/virtio/virtgpu_vq.c    | 2 ++
>  2 files changed, 5 insertions(+)
> 
> diff --git a/drivers/gpu/drm/virtio/virtgpu_ioctl.c b/drivers/gpu/drm/virtio/virtgpu_ioctl.c
> index 88c1ed57a3c5..502f5f7c2298 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_ioctl.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_ioctl.c
> @@ -542,6 +542,9 @@ static int virtio_gpu_get_caps_ioctl(struct drm_device *dev,
>  	if (!ret)
>  		return -EBUSY;
>  
> +	/* is_valid check must proceed before copy of the cache entry. */
> +	virt_rmb();

I don't think you need virt_rmb() here.  This isn't guest <=> host
communication, so a normal barrier should do.

The other three fixes are queued up for drm-misc-next.

cheers,
  Gerd