Re: [dpdk-dev] [PATCH] virtio: fix rx ring descriptor starvation

["Xie, Huawei" <huawei.xie@intel.com>]

On 11/13/2015 5:33 PM, Tom Kiely wrote:
> If all rx descriptors are processed while transient
> mbuf exhaustion is present, the rx ring ends up with
> no available descriptors. Thus no packets are received
> on that ring. Since descriptor refill is performed post
> rx descriptor processing, in this case no refill is
> ever subsequently performed resulting in permanent rx
> traffic drop.
>
> Signed-off-by: Tom Kiely <tkiely@brocade.com>
> ---
>  drivers/net/virtio/virtio_rxtx.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
> index 5770fa2..a95e234 100644
> --- a/drivers/net/virtio/virtio_rxtx.c
> +++ b/drivers/net/virtio/virtio_rxtx.c
> @@ -586,7 +586,8 @@ virtio_recv_pkts(void *rx_queue, struct rte_mbuf **rx_pkts, uint16_t nb_pkts)
>  	if (likely(num > DESC_PER_CACHELINE))
>  		num = num - ((rxvq->vq_used_cons_idx + num) % DESC_PER_CACHELINE);
>  
> -	if (num == 0)
> +	/* Refill free descriptors even if no pkts recvd */
> +	if (num == 0 && virtqueue_full(rxvq))
Should the return condition be that no used buffers and we have avail
descs in avail ring, i.e,
    num == 0 && rxvq->vq_free_cnt != rxvq->vq_nentries

rather than
    num == 0 && rxvq->vq_free_cnt == 0
?
>  		return 0;
>  
>  	num = virtqueue_dequeue_burst_rx(rxvq, rcv_pkts, len, num);
> @@ -683,7 +684,8 @@ virtio_recv_mergeable_pkts(void *rx_queue,
>  
>  	virtio_rmb();
>  
> -	if (nb_used == 0)
> +	/* Refill free descriptors even if no pkts recvd */
> +	if (nb_used == 0 && virtqueue_full(rxvq))
>  		return 0;
>  
>  	PMD_RX_LOG(DEBUG, "used:%d\n", nb_used);