[PATH 2/2] vhost: fix slot index calculation in async vhost

[Maxime Coquelin]

On 8/22/22 06:31, Cheng Jiang wrote:
> When the packet receiving failure and the DMA ring full occur
> simultaneously in the asynchronous vhost, the slot_idx needs to be
> reduced by 1. For packed virtqueue, the slot index should be

s/reduced/decreased/

> ring_size - 1, if the slot_idx is currently 0, since the ring size is
> not necessarily the power of 2.

This is a fix, so fixes tag and stable at dpdk.org is required.

> Signed-off-by: Cheng Jiang <cheng1.jiang at intel.com>
> ---
>   lib/vhost/virtio_net.c | 16 ++++++++++++++--
>   1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
> index bfc6d65b7c..f804bce0bd 100644
> --- a/lib/vhost/virtio_net.c
> +++ b/lib/vhost/virtio_net.c
> @@ -3462,6 +3462,7 @@ virtio_dev_tx_async_split(struct virtio_net *dev, struct vhost_virtqueue *vq,
>   				allocerr_warned = true;
>   			}
>   			dropped = true;
> +			slot_idx--;
>   			break;
>   		}
>   
> @@ -3652,6 +3653,12 @@ virtio_dev_tx_async_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
>   		if (unlikely(virtio_dev_tx_async_single_packed(dev, vq, mbuf_pool, pkt,
>   				slot_idx, legacy_ol_flags))) {
>   			rte_pktmbuf_free_bulk(&pkts_prealloc[pkt_idx], count - pkt_idx);
> +
> +			if (slot_idx == 0)
> +				slot_idx = vq->size - 1;
> +			else
> +				slot_idx--;
> +
>   			break;
>   		}
>   
> @@ -3679,8 +3686,13 @@ virtio_dev_tx_async_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
>   			async->buffer_idx_packed += vq->size - pkt_err;
>   
>   		while (pkt_err-- > 0) {
> -			rte_pktmbuf_free(pkts_info[slot_idx % vq->size].mbuf);
> -			slot_idx--;
> +			rte_pktmbuf_free(pkts_info[slot_idx].mbuf);
> +			descs_err += pkts_info[slot_idx].descs;
> +
> +			if (slot_idx == 0)
> +				slot_idx = vq->size - 1;
> +			else
> +				slot_idx--;
>   		}
>   
>   		/* recover available ring */