[dpdk-dev] [PATCH v5] virtio: Fix vring entry number issue

[Ouyang Changchun <changchun.ouyang@intel.com>]

  Fix one issue in virtio TX: it needs one more vring descriptor to hold the virtio
  header when transmitting packets, it is used later to determine whether to free
  more entries from used vring.
  It fixes failing to transmit any packet with 1 segment in the circumstance of only
  1 descriptor in the vring free list.

Signed-off-by: Changchun Ouyang <changchun.ouyang@intel.com>
Reviewed-by: Olivier Matz <olivier.matz@6wind.com>
---

-V5 change:
  Use 'likely' to hint compiler to generate favorable codes.

-V4 change:
  Relocate those changelog and annotation here.
 
-V3 change:
  Resolve compilation issue due to mbuf re-structure.
 
-V2 change:
  Refine description, 'vring descriptor' is more accurate than 'vring entry';
  Accordingly update test condition (it needs one more descriptor for virtio header) to
  put packets to transmit queue.
 
-V1 change:
  Fix the vring descriptor issue in virtio TX, it needs one more vring descriptor to hold the
  virtio header.

 lib/librte_pmd_virtio/virtio_rxtx.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/lib/librte_pmd_virtio/virtio_rxtx.c b/lib/librte_pmd_virtio/virtio_rxtx.c
index ad6401c..3f6bad2 100644
--- a/lib/librte_pmd_virtio/virtio_rxtx.c
+++ b/lib/librte_pmd_virtio/virtio_rxtx.c
@@ -699,7 +699,8 @@ virtio_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts, uint16_t nb_pkts)
 	num = (uint16_t)(likely(nb_used < VIRTIO_MBUF_BURST_SZ) ? nb_used : VIRTIO_MBUF_BURST_SZ);
 
 	while (nb_tx < nb_pkts) {
-		int need = tx_pkts[nb_tx]->nb_segs - txvq->vq_free_cnt;
+		/* Need one more descriptor for virtio header. */
+		int need = tx_pkts[nb_tx]->nb_segs - txvq->vq_free_cnt + 1;
 		int deq_cnt = RTE_MIN(need, (int)num);
 
 		num -= (deq_cnt > 0) ? deq_cnt : 0;
@@ -708,7 +709,12 @@ virtio_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts, uint16_t nb_pkts)
 			deq_cnt--;
 		}
 
-		if (tx_pkts[nb_tx]->nb_segs <= txvq->vq_free_cnt) {
+		need = (int)tx_pkts[nb_tx]->nb_segs - txvq->vq_free_cnt + 1;
+		/*
+		 * Zero or negative value indicates it has enough free
+		 * descriptors to use for transmitting.
+		 */
+		if (likely(need <= 0)) {
 			txm = tx_pkts[nb_tx];
 			/* Enqueue Packet buffers */
 			error = virtqueue_enqueue_xmit(txvq, txm);
-- 
1.8.4.2