[dpdk-dev] [PATCH v3 03/15] vhost: add batch enqueue function for packed ring

[Liu, Yong]

> -----Original Message-----
> From: Mattias Rönnblom [mailto:hofors at lysator.liu.se]
> Sent: Thursday, September 26, 2019 3:31 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>;
> stephen at networkplumber.org; gavin.hu at arm.com
> Cc: dev at dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v3 03/15] vhost: add batch enqueue function
> for packed ring
> 
> On 2019-09-25 19:13, Marvin Liu wrote:
> > Batch enqueue function will first check whether descriptors are cache
> > aligned. It will also check prerequisites in the beginning. Batch
> > enqueue function not support chained mbufs, single packet enqueue
> > function will handle it.
> >
> > 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 4cba8c5ef..e241436c7 100644
> > --- a/lib/librte_vhost/vhost.h
> > +++ b/lib/librte_vhost/vhost.h
> > @@ -39,6 +39,10 @@
> >
> >   #define VHOST_LOG_CACHE_NR 32
> >
> > +#define PACKED_BATCH_SIZE (RTE_CACHE_LINE_SIZE / \
> > +			    sizeof(struct vring_packed_desc))
> > +#define PACKED_BATCH_MASK (PACKED_BATCH_SIZE - 1)
> > +
> >   #ifdef SUPPORT_GCC_UNROLL_PRAGMA
> >   #define UNROLL_PRAGMA_PARAM "GCC unroll 4"
> >   #endif
> > diff --git a/lib/librte_vhost/virtio_net.c
> b/lib/librte_vhost/virtio_net.c
> > index 520c4c6a8..5e08f7d9b 100644
> > --- a/lib/librte_vhost/virtio_net.c
> > +++ b/lib/librte_vhost/virtio_net.c
> > @@ -883,6 +883,86 @@ virtio_dev_rx_split(struct virtio_net *dev, struct
> vhost_virtqueue *vq,
> >   	return pkt_idx;
> >   }
> >
> > +static __rte_unused int
> > +virtio_dev_rx_batch_packed(struct virtio_net *dev, struct
> vhost_virtqueue *vq,
> > +	 struct rte_mbuf **pkts)
> > +{
> > +	bool wrap_counter = vq->avail_wrap_counter;
> > +	struct vring_packed_desc *descs = vq->desc_packed;
> > +	uint16_t avail_idx = vq->last_avail_idx;
> > +	uint64_t desc_addrs[PACKED_BATCH_SIZE];
> > +	struct virtio_net_hdr_mrg_rxbuf *hdrs[PACKED_BATCH_SIZE];
> > +	uint32_t buf_offset = dev->vhost_hlen;
> > +	uint64_t lens[PACKED_BATCH_SIZE];
> > +	uint16_t i;
> > +
> > +	if (unlikely(avail_idx & PACKED_BATCH_MASK))
> > +		return -1;
> 
> Does this really generate better code than just "avail_idx <
> PACKED_BATCH_SIZE"? and+jne vs cmp+jbe.

Hi Mattias,
This comparison is to check whether descriptor location is cache aligned. 
In x86 cache line size is 64 bytes, so here mask is 0x3. This check will be and + test + je which is very simple. 
Most of times the cost of execution will be eliminated as the result can be predicted.

Thanks,
Marvin

> 
> > +
> > +	if (unlikely((avail_idx + PACKED_BATCH_SIZE) > vq->size))
> > +		return -1;
> > +
> > +	UNROLL_PRAGMA(UNROLL_PRAGMA_PARAM)
> > +	for (i = 0; i < PACKED_BATCH_SIZE; i++) {
> > +		if (unlikely(pkts[i]->next != NULL))
> > +			return -1;
> > +		if (unlikely(!desc_is_avail(&descs[avail_idx + i],
> > +					    wrap_counter)))
> > +			return -1;
> > +	}
> > +
> > +	rte_smp_rmb();
> > +
> > +	UNROLL_PRAGMA(UNROLL_PRAGMA_PARAM)
> > +	for (i = 0; i < PACKED_BATCH_SIZE; i++)
> > +		lens[i] = descs[avail_idx + i].len;
> > +
> > +	UNROLL_PRAGMA(UNROLL_PRAGMA_PARAM)
> > +	for (i = 0; i < PACKED_BATCH_SIZE; i++) {
> > +		if (unlikely(pkts[i]->pkt_len > (lens[i] - buf_offset)))
> > +			return -1;
> > +	}
> > +
> > +	UNROLL_PRAGMA(UNROLL_PRAGMA_PARAM)
> > +	for (i = 0; i < PACKED_BATCH_SIZE; i++)
> > +		desc_addrs[i] = vhost_iova_to_vva(dev, vq,
> > +						  descs[avail_idx + i].addr,
> > +						  &lens[i],
> > +						  VHOST_ACCESS_RW);
> > +	UNROLL_PRAGMA(UNROLL_PRAGMA_PARAM)
> > +	for (i = 0; i < PACKED_BATCH_SIZE; i++) {
> > +		if (unlikely(lens[i] != descs[avail_idx + i].len))
> > +			return -1;
> > +	}
> > +
> > +	UNROLL_PRAGMA(UNROLL_PRAGMA_PARAM)
> > +	for (i = 0; i < PACKED_BATCH_SIZE; i++) {
> > +		rte_prefetch0((void *)(uintptr_t)desc_addrs[i]);
> > +		hdrs[i] = (struct virtio_net_hdr_mrg_rxbuf *)
> > +					(uintptr_t)desc_addrs[i];
> > +		lens[i] = pkts[i]->pkt_len + dev->vhost_hlen;
> > +	}
> > +
> > +	UNROLL_PRAGMA(UNROLL_PRAGMA_PARAM)
> > +	for (i = 0; i < PACKED_BATCH_SIZE; i++)
> > +		virtio_enqueue_offload(pkts[i], &hdrs[i]->hdr);
> > +
> > +	vq->last_avail_idx += PACKED_BATCH_SIZE;
> > +	if (vq->last_avail_idx >= vq->size) {
> > +		vq->last_avail_idx -= vq->size;
> > +		vq->avail_wrap_counter ^= 1;
> > +	}
> > +
> > +	UNROLL_PRAGMA(UNROLL_PRAGMA_PARAM)
> > +	for (i = 0; i < PACKED_BATCH_SIZE; i++) {
> > +		rte_memcpy((void *)(uintptr_t)(desc_addrs[i] + buf_offset),
> > +			   rte_pktmbuf_mtod_offset(pkts[i], void *, 0),
> > +			   pkts[i]->pkt_len);
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> >   static __rte_unused int16_t
> >   virtio_dev_rx_single_packed(struct virtio_net *dev, struct
> vhost_virtqueue *vq,
> >   	struct rte_mbuf *pkt)
> >