[PATCH v2 2/2] vhost: fix slot index calculation in async vhost

[Cheng Jiang <cheng1.jiang@intel.com>]

When the packet receiving failure and the DMA ring full occur
simultaneously in the asynchronous vhost, the slot_idx needs to be
decreased by 1. For packed virtqueue, the slot index should be
ring_size - 1, if the slot_idx is currently 0, since the ring size is
not necessarily the power of 2.

Fixes: 84d5204310d7 ("vhost: support async dequeue for split ring")
Fixes: fe8477ebbd94 ("vhost: support async packed ring dequeue")
Cc: stable@dpdk.org

Signed-off-by: Cheng Jiang <cheng1.jiang@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 457ac2e92a..efebd063d7 100644
--- a/lib/vhost/virtio_net.c
+++ b/lib/vhost/virtio_net.c
@@ -3457,6 +3457,7 @@ virtio_dev_tx_async_split(struct virtio_net *dev, struct vhost_virtqueue *vq,
 				allocerr_warned = true;
 			}
 			dropped = true;
+			slot_idx--;
 			break;
 		}
 
@@ -3647,6 +3648,12 @@ 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;
 		}
 
@@ -3674,8 +3681,13 @@ 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 */
-- 
2.35.1