[PATCH] net/mlx5: do not poll CQEs when no available elts

[Gavin Hu <gahu@nvidia.com>]

In certain situations, the receive queue (rxq) fails to replenish its
internal ring with memory buffers (mbufs) from the pool. This can happen
when the pool has a limited number of mbufs allocated, and the user
application holds incoming packets for an extended period, resulting in a
delayed release of mbufs. Consequently, the pool becomes depleted,
preventing the rxq from replenishing from it.

There was a bug in the behavior of the vectorized rxq_cq_process_v routine,
which handled completion queue entries (CQEs) in batches of four. This
routine consistently accessed four mbufs from the internal queue ring,
regardless of whether they had been replenished. As a result, it could
access mbufs that no longer belonged to the poll mode driver (PMD).

The fix involves checking if there are four replenished mbufs available
before allowing rxq_cq_process_v to handle the batch. Once replenishment
succeeds during the polling process, the routine will resume its operation.

Fixes: 1ded26239aa0 ("net/mlx5: refactor vectorized Rx")
Cc: stable@dpdk.org

Reported-by: Changqi Dingluo <dingluochangqi.ck@bytedance.com>
Signed-off-by: Gavin Hu <gahu@nvidia.com>
---
 drivers/net/mlx5/mlx5_rxtx_vec.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/mlx5/mlx5_rxtx_vec.c b/drivers/net/mlx5/mlx5_rxtx_vec.c
index 1872bf310c..1b701801c5 100644
--- a/drivers/net/mlx5/mlx5_rxtx_vec.c
+++ b/drivers/net/mlx5/mlx5_rxtx_vec.c
@@ -325,6 +325,9 @@ rxq_burst_v(struct mlx5_rxq_data *rxq, struct rte_mbuf **pkts,
 	/* Not to cross queue end. */
 	pkts_n = RTE_MIN(pkts_n, q_n - elts_idx);
 	pkts_n = RTE_MIN(pkts_n, q_n - cq_idx);
+	/* Not to move past the allocated mbufs. */
+	pkts_n = RTE_MIN(pkts_n, RTE_ALIGN_FLOOR(rxq->rq_ci - rxq->rq_pi,
+						MLX5_VPMD_DESCS_PER_LOOP));
 	if (!pkts_n) {
 		*no_cq = !rcvd_pkt;
 		return rcvd_pkt;
-- 
2.18.2