Re: [dpdk-dev] [PATCH v4] virtio: Fix vring entry number issue

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

Hi Huawei,
I have v5 patch for this comment.
Thanks,
Changchun

> -----Original Message-----
> From: Xie, Huawei
> Sent: Wednesday, October 15, 2014 12:26 AM
> To: Ouyang, Changchun; dev@dpdk.org
> Subject: RE: [dpdk-dev] [PATCH v4] virtio: Fix vring entry number issue
> 
> 
> 
> > -----Original Message-----
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ouyang
> Changchun
> > Sent: Tuesday, October 14, 2014 9:04 AM
> > To: dev@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@intel.com>
> > Reviewed-by: Olivier Matz <olivier.matz@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