vdpa/mlx5: Use write memory barrier after updating CQ index

Make sure to put dma write memory barrier after updating CQ consumer
index so the hardware knows that there are available CQE slots in the
queue.

Failure to do this can cause the update of the RX doorbell record to get
updated before the CQ consumer index resulting in CQ overrun.

Fixes: 1a86b377 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20201209140004.15892-1-elic@nvidia.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vdpa/mlx5/net/mlx5_vnet.c

@@ -464,6 +464,11 @@ static int mlx5_vdpa_poll_one(struct mlx5_vdpa_cq *vcq)
 static void mlx5_vdpa_handle_completions(struct mlx5_vdpa_virtqueue *mvq, int num)
 {
 	mlx5_cq_set_ci(&mvq->cq.mcq);
+
+	/* make sure CQ cosumer update is visible to the hardware before updating
+	 * RX doorbell record.
+	 */
+	dma_wmb();
 	rx_post(&mvq->vqqp, num);
 	if (mvq->event_cb.callback)
 		mvq->event_cb.callback(mvq->event_cb.private);