[dpdk-dev] [PATCH v2 08/16] vhost: add flush function for burst enqueue

[Gavin Hu (Arm Technology China)]

Hi Marvin,

> -----Original Message-----
> From: Liu, Yong <yong.liu at intel.com>
> Sent: Wednesday, September 25, 2019 2:52 PM
> To: Gavin Hu (Arm Technology China) <Gavin.Hu at arm.com>;
> 'maxime.coquelin at redhat.com' <maxime.coquelin at redhat.com>; Bie,
> Tiwei <tiwei.bie at intel.com>; Wang, Zhihong <zhihong.wang at intel.com>
> Cc: 'dev at dpdk.org' <dev at dpdk.org>; nd <nd at arm.com>
> Subject: RE: [dpdk-dev] [PATCH v2 08/16] vhost: add flush function for burst
> enqueue
> 
> > -----Original Message-----
> > From: Liu, Yong
> > Sent: Wednesday, September 25, 2019 1:38 PM
> > To: Gavin Hu (Arm Technology China) <Gavin.Hu at arm.com>;
> > maxime.coquelin at redhat.com; Bie, Tiwei <tiwei.bie at intel.com>; Wang,
> Zhihong
> > <zhihong.wang at intel.com>
> > Cc: dev at dpdk.org; nd <nd at arm.com>
> > Subject: RE: [dpdk-dev] [PATCH v2 08/16] vhost: add flush function for
> > burst enqueue
> >
> >
> >
> > > -----Original Message-----
> > > From: Gavin Hu (Arm Technology China) [mailto:Gavin.Hu at arm.com]
> > > Sent: Wednesday, September 25, 2019 11:38 AM
> > > To: Liu, Yong <yong.liu at intel.com>; maxime.coquelin at redhat.com; Bie,
> > Tiwei
> > > <tiwei.bie at intel.com>; Wang, Zhihong <zhihong.wang at intel.com>
> > > Cc: dev at dpdk.org; nd <nd at arm.com>
> > > Subject: RE: [dpdk-dev] [PATCH v2 08/16] vhost: add flush function for
> > > burst enqueue
> > >
> > > Hi Marvin,
> > >
> > > One typo and one comment about the barrier.
> > >
> > > /Gavin
> > >
> > > > -----Original Message-----
> > > > From: dev <dev-bounces at dpdk.org> On Behalf Of Marvin Liu
> > > > Sent: Friday, September 20, 2019 12:37 AM
> > > > To: maxime.coquelin at redhat.com; tiwei.bie at intel.com;
> > > > zhihong.wang at intel.com
> > > > Cc: dev at dpdk.org; Marvin Liu <yong.liu at intel.com>
> > > > Subject: [dpdk-dev] [PATCH v2 08/16] vhost: add flush function for
> > burst
> > > > enqueue
> > > >
> > > > Flush used flags when burst enqueue function is finished. Descriptor's
> > > > flags are pre-calculated as them will be reset by vhost.
> > > s/them/they
> > >
> >
> > Thanks.
> >
> > > >
> > > > Signed-off-by: Marvin Liu <yong.liu at intel.com>
> > > >
> > > > diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
> > > > index 000648dd4..9c42c7db0 100644
> > > > --- a/lib/librte_vhost/vhost.h
> > > > +++ b/lib/librte_vhost/vhost.h
> > > > @@ -39,6 +39,9 @@
> > > >
> > > >  #define VHOST_LOG_CACHE_NR 32
> > > >
> > > > +#define VIRTIO_RX_USED_FLAG  (0ULL | VRING_DESC_F_AVAIL |
> > > > VRING_DESC_F_USED \
> > > > +				| VRING_DESC_F_WRITE)
> > > > +#define VIRTIO_RX_USED_WRAP_FLAG (VRING_DESC_F_WRITE)
> > > >  #define PACKED_DESCS_BURST (RTE_CACHE_LINE_SIZE / \
> > > >  			    sizeof(struct vring_packed_desc))
> > > >
> > > > diff --git a/lib/librte_vhost/virtio_net.c
> > > b/lib/librte_vhost/virtio_net.c
> > > > index e2787b72e..8e4036204 100644
> > > > --- a/lib/librte_vhost/virtio_net.c
> > > > +++ b/lib/librte_vhost/virtio_net.c
> > > > @@ -169,6 +169,51 @@ update_shadow_packed(struct
> vhost_virtqueue
> > > > *vq,
> > > >  	vq->shadow_used_packed[i].count = count;
> > > >  }
> > > >
> > > > +static __rte_always_inline void
> > > > +flush_burst_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
> > > > +	uint64_t *lens, uint16_t *ids, uint16_t flags)
> > > > +{
> > > > +	uint16_t i;
> > > > +
> > > > +	UNROLL_PRAGMA(PRAGMA_PARAM)
> > > > +	for (i = 0; i < PACKED_DESCS_BURST; i++) {
> > > > +		vq->desc_packed[vq->last_used_idx + i].id = ids[i];
> > > > +		vq->desc_packed[vq->last_used_idx + i].len = lens[i];
> > > > +	}
> > > > +
> > > > +	UNROLL_PRAGMA(PRAGMA_PARAM)
> > > > +	for (i = 0; i < PACKED_DESCS_BURST; i++) {
> > > > +		rte_smp_wmb();
> > > Should this rte_smp_wmb() be moved above the loop? It guarantees the
> > > orderings of updates of id, len happens before the flags,
> > > But all the flags of different descriptors should not be ordered.
> > >
> > Hi Gavin,
> > For each descriptor, virtio driver will first check flags and then check
> > read barrier, at the last driver will read id and length.
> > So wmb here is to guarantee that id and length are updated before flags.
> > And afterwards wmb is to guarantee the sequence.
> >
> Gavin,
> Checked with master branch, flags store sequence is not needed.
> But in my environment, performance will be a litter better if ordered flags
> store.
> I think it may be harmless to place wmb here. How about your idea?
The smp barrier on x86 is a compiler barrier only, it ensure data consistency, it will not help performance,
The slight better performance should come from run-to-run variances or system noise or sth else. 
The barrier will dampen the performance on weak memory ordered platforms, like aarch64. 
/Gavin
> 
> > Thanks,
> > Marvin
> >
> > > > +		vq->desc_packed[vq->last_used_idx + i].flags = flags;
> > > > +	}
> > > > +
> > > > +	vhost_log_cache_used_vring(dev, vq, vq->last_used_idx *
> > > > +				   sizeof(struct vring_packed_desc),
> > > > +				   sizeof(struct vring_packed_desc) *
> > > > +				   PACKED_DESCS_BURST);
> > > > +	vhost_log_cache_sync(dev, vq);
> > > > +
> > > > +	vq->last_used_idx += PACKED_DESCS_BURST;
> > > > +	if (vq->last_used_idx >= vq->size) {
> > > > +		vq->used_wrap_counter ^= 1;
> > > > +		vq->last_used_idx -= vq->size;
> > > > +	}
> > > > +}
> > > > +
> > > > +static __rte_always_inline void
> > > > +flush_enqueue_burst_packed(struct virtio_net *dev, struct
> > > > vhost_virtqueue *vq,
> > > > +	uint64_t *lens, uint16_t *ids)
> > > > +{
> > > > +	uint16_t flags = 0;
> > > > +
> > > > +	if (vq->used_wrap_counter)
> > > > +		flags = VIRTIO_RX_USED_FLAG;
> > > > +	else
> > > > +		flags = VIRTIO_RX_USED_WRAP_FLAG;
> > > > +
> > > > +	flush_burst_packed(dev, vq, lens, ids, flags);
> > > > +}
> > > > +
> > > >  static __rte_always_inline void
> > > >  update_enqueue_shadow_packed(struct vhost_virtqueue *vq,
> uint16_t
> > > > desc_idx,
> > > >  	uint32_t len, uint16_t count)
> > > > @@ -950,6 +995,7 @@ virtio_dev_rx_burst_packed(struct virtio_net
> *dev,
> > > > struct vhost_virtqueue *vq,
> > > >  	struct virtio_net_hdr_mrg_rxbuf *hdrs[PACKED_DESCS_BURST];
> > > >  	uint32_t buf_offset = dev->vhost_hlen;
> > > >  	uint64_t lens[PACKED_DESCS_BURST];
> > > > +	uint16_t ids[PACKED_DESCS_BURST];
> > > >
> > > >  	uint16_t i;
> > > >
> > > > @@ -1013,6 +1059,12 @@ virtio_dev_rx_burst_packed(struct
> virtio_net
> > > > *dev, struct vhost_virtqueue *vq,
> > > >  			   pkts[i]->pkt_len);
> > > >  	}
> > > >
> > > > +	UNROLL_PRAGMA(PRAGMA_PARAM)
> > > > +	for (i = 0; i < PACKED_DESCS_BURST; i++)
> > > > +		ids[i] = descs[avail_idx + i].id;
> > > > +
> > > > +	flush_enqueue_burst_packed(dev, vq, lens, ids);
> > > > +
> > > >  	return 0;
> > > >  }
> > > >
> > > > --
> > > > 2.17.1