drm/virtio: Add memory barriers for capset cache.

[ Upstream commit 9ff3a5c8 ]

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 corresponding memory barriers.
Signed-off-by: David Riley <davidriley@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190610211810.253227-5-davidriley@chromium.orgSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/gpu/drm/virtio/virtgpu_ioctl.c

@@ -528,6 +528,9 @@ 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. */
+	smp_rmb();
+
 	ptr = cache_ent->caps_cache;
 
 copy_exit:

drivers/gpu/drm/virtio/virtgpu_vq.c

@@ -588,6 +588,8 @@ static void virtio_gpu_cmd_capset_cb(struct virtio_gpu_device *vgdev,
 		    cache_ent->id == le32_to_cpu(cmd->capset_id)) {
 			memcpy(cache_ent->caps_cache, resp->capset_data,
 			       cache_ent->size);
+			/* Copy must occur before is_valid is signalled. */
+			smp_wmb();
 			atomic_set(&cache_ent->is_valid, 1);
 			break;
 		}