Re: [PATCH] vhost: avoid memory barriers when no descriptors dequeued

[Tiwei Bie <tiwei.bie@intel.com>]

On Fri, Oct 19, 2018 at 04:00:58PM +0200, Maxime Coquelin wrote:
> In both split and packed dequeue paths, flush_shadow_used_ring
> and vhost_ring_call variants gets called even if not packets
> have been dequeued, and so no descriptors updates happened.
> 
> It has an impact on CPU pipeline, as memory barriers are used
> in these functions.
> 
> This patch don't call these functions if no descriptors have
> been dequeued. The performance gain with split ring when
> dequeue zero-copy is disabled should be null, but should be
> noticeable with packed ring or dequeue zero-copy enabled.
> 
> Fixes: ae999ce49dcb ("vhost: add Tx support for packed ring")
> Fixes: 915cf9404225 ("vhost: use shadow used ring in dequeue path")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
>  lib/librte_vhost/virtio_net.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
[...]
>  
> -	if (likely(dev->dequeue_zero_copy == 0)) {
> +	if (likely(dev->dequeue_zero_copy == 0 && i != 0)) {
>  		do_data_copy_dequeue(vq);
>  		if (unlikely(i < count))
>  			vq->shadow_used_idx = i;

When i is 0, we may need to update vq->shadow_used_idx to 0,
e.g. when error happens after update_shadow_used_ring_split()
in the first iteration of the loop.

> @@ -1475,8 +1477,10 @@ virtio_dev_tx_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
>  			}
>  		}
>  
> -		flush_shadow_used_ring_packed(dev, vq);
> -		vhost_vring_call_packed(dev, vq);
> +		if (likely(vq->shadow_used_idx)) {
> +			flush_shadow_used_ring_packed(dev, vq);
> +			vhost_vring_call_packed(dev, vq);
> +		}
>  	}
>  
>  	VHOST_LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__);
> @@ -1550,7 +1554,7 @@ virtio_dev_tx_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
>  		}
>  	}
>  
> -	if (likely(dev->dequeue_zero_copy == 0)) {
> +	if (likely(dev->dequeue_zero_copy == 0 && i != 0)) {

Ditto

>  		do_data_copy_dequeue(vq);
>  		if (unlikely(i < count))
>  			vq->shadow_used_idx = i;
> -- 
> 2.17.1
>