[PATCH] net/virtio: Fix check of threshold for Tx freeing

[Hengqi Chen <hengqi.chen@gmail.com>]

Like most dirvers, make the fast path of virtio_xmit_cleanup() behave as
described by the comments of rte_eth_txconf::tx_free_thresh ([0]):
    Start freeing Tx buffers if there are
    less free descriptors than this value.

The rationale behind this change is that:
  * vq->vq_nentries is set during device probe with the queue size specified
    by vhost backend, this value does not reflect the real nb_tx_desc
  * the real available tx desc is set to vq->vq_free_cnt via the nb_tx_desc
    param of rte_eth_tx_queue_setup() API
  * so `nb_used > vq->vq_nentries - vq->vq_free_thresh` could never be true
    if say nb_tx_desc=2048, vq->vq_nentries=4096, vq->vq_free_thresh=32,
    see bug report 1716 ([1]) for details.

Bugzilla ID: 1716

  [0]: https://github.com/DPDK/dpdk/commit/72514b5d5543
  [1]: https://bugs.dpdk.org/show_bug.cgi?id=1716

Signed-off-by: Baoyuan Li <updoing@sina.com>
Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
---
 drivers/net/virtio/virtio_rxtx.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
index edecd2011f..ab97f03d7d 100644
--- a/drivers/net/virtio/virtio_rxtx.c
+++ b/drivers/net/virtio/virtio_rxtx.c
@@ -1873,7 +1873,7 @@ virtio_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts, uint16_t nb_pkts)
 
 	nb_used = virtqueue_nused(vq);
 
-	if (likely(nb_used > vq->vq_nentries - vq->vq_free_thresh))
+	if (likely(vq->vq_free_cnt < vq->vq_free_thresh))
 		virtio_xmit_cleanup(vq, nb_used);
 
 	for (nb_tx = 0; nb_tx < nb_pkts; nb_tx++) {
-- 
2.43.5