Re: [PATCH] vdpa/mlx5: Use write memory barrier after updating CQ index

[Michael S. Tsirkin]

On Sun, Dec 06, 2020 at 12:57:19PM +0200, Eli Cohen wrote:
> Make sure to put 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.
> 
> Change-Id: Ib0ae4c118cce524c9f492b32569179f3c1f04cc1
> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
> Signed-off-by: Eli Cohen <elic@xxxxxxxxxx>

Aren't both memory writes? And given that, isn't dma_wmb() sufficient
here?


> ---
>  drivers/vdpa/mlx5/net/mlx5_vnet.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> index 1f4089c6f9d7..295f46eea2a5 100644
> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> @@ -478,6 +478,11 @@ 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.
> +	 */
> +	wmb();
>  	rx_post(&mvq->vqqp, num);
>  	if (mvq->event_cb.callback)
>  		mvq->event_cb.callback(mvq->event_cb.private);
> -- 
> 2.27.0