> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Ouyang Changchun
> Sent: Tuesday, October 14, 2014 9:04 AM
> To: dev at dpdk.org
> Subject: [dpdk-dev] [PATCH v4] virtio: Fix vring entry number issue
>
> 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 at intel.com>
> Reviewed-by: Olivier Matz <olivier.matz at 6wind.com>
> ---
> -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 0b10108..ac44c65 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]->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]->nb_segs - txvq->vq_free_cnt + 1;
> + /*
> + * Zero or negative value indicates it has enough free
> + * decriptors to transmit.
> + */
> + if (need <= 0) {
Could we put a branch hint to favor the fast path?
> txm = tx_pkts[nb_tx];
> /* Enqueue Packet buffers */
> error = virtqueue_enqueue_xmit(txvq, txm);
> --
> 1.8.4.2
