-- V1 change:
  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.

-- 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.

Signed-off-by: Changchun Ouyang <changchun.ouyang at intel.com>
---
 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 0b10108..c736362 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]->pkt.nb_segs - txvq->vq_free_cnt;
+               /* Need one more descriptor for virtio header. */
+               int need = tx_pkts[nb_tx]->pkt.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]->pkt.nb_segs <= txvq->vq_free_cnt) {
+               need = (int)tx_pkts[nb_tx]->pkt.nb_segs - txvq->vq_free_cnt + 1;
+               /*
+                * Zero or negative value indicates it has enough free
+                * decriptors to transmit.
+                */
+               if (need <= 0) {
                        txm = tx_pkts[nb_tx];
                        /* Enqueue Packet buffers */
                        error = virtqueue_enqueue_xmit(txvq, txm);
-- 
1.8.4.2

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