[dpdk-dev] [PATCH 2/3] vhost: optimize dequeue for small packets

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

Both current kernel virtio driver and DPDK virtio driver use at least
2 desc buffer for Tx: the first for storing the header, and the others
for storing the data.

Therefore, we could fetch the first data desc buf before the main loop,
and do the copy first before the check of "are we done yet?". This
could save one check for small packets, that just have one data desc
buffer and need one mbuf to store it.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
---
 lib/librte_vhost/vhost_rxtx.c | 52 ++++++++++++++++++++++++++++++-------------
 1 file changed, 36 insertions(+), 16 deletions(-)

diff --git a/lib/librte_vhost/vhost_rxtx.c b/lib/librte_vhost/vhost_rxtx.c
index 2c3b810..34d6ed1 100644
--- a/lib/librte_vhost/vhost_rxtx.c
+++ b/lib/librte_vhost/vhost_rxtx.c
@@ -753,18 +753,48 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		return -1;
 
 	desc_addr = gpa_to_vva(dev, desc->addr);
-	rte_prefetch0((void *)(uintptr_t)desc_addr);
-
-	/* Retrieve virtio net header */
 	hdr = (struct virtio_net_hdr *)((uintptr_t)desc_addr);
-	desc_avail  = desc->len - dev->vhost_hlen;
-	desc_offset = dev->vhost_hlen;
+	rte_prefetch0(hdr);
+
+	/*
+	 * Both current kernel virio driver and DPDK virtio driver
+	 * use at least 2 desc bufferr for Tx: the first for storing
+	 * the header, and others for storing the data.
+	 */
+	if (likely(desc->len == dev->vhost_hlen)) {
+		desc = &vq->desc[desc->next];
+
+		desc_addr = gpa_to_vva(dev, desc->addr);
+		rte_prefetch0((void *)(uintptr_t)desc_addr);
+
+		desc_offset = 0;
+		desc_avail  = desc->len;
+		nr_desc    += 1;
+
+		PRINT_PACKET(dev, (uintptr_t)desc_addr, desc->len, 0);
+	} else {
+		desc_avail  = desc->len - dev->vhost_hlen;
+		desc_offset = dev->vhost_hlen;
+	}
 
 	mbuf_offset = 0;
 	mbuf_avail  = m->buf_len - RTE_PKTMBUF_HEADROOM;
-	while (desc_avail != 0 || (desc->flags & VRING_DESC_F_NEXT) != 0) {
+	while (1) {
+		cpy_len = RTE_MIN(desc_avail, mbuf_avail);
+		rte_memcpy(rte_pktmbuf_mtod_offset(cur, void *, mbuf_offset),
+			(void *)((uintptr_t)(desc_addr + desc_offset)),
+			cpy_len);
+
+		mbuf_avail  -= cpy_len;
+		mbuf_offset += cpy_len;
+		desc_avail  -= cpy_len;
+		desc_offset += cpy_len;
+
 		/* This desc reaches to its end, get the next one */
 		if (desc_avail == 0) {
+			if ((desc->flags & VRING_DESC_F_NEXT) == 0)
+				break;
+
 			if (unlikely(desc->next >= vq->size ||
 				     ++nr_desc >= vq->size))
 				return -1;
@@ -800,16 +830,6 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vhost_virtqueue *vq,
 			mbuf_offset = 0;
 			mbuf_avail  = cur->buf_len - RTE_PKTMBUF_HEADROOM;
 		}
-
-		cpy_len = RTE_MIN(desc_avail, mbuf_avail);
-		rte_memcpy(rte_pktmbuf_mtod_offset(cur, void *, mbuf_offset),
-			(void *)((uintptr_t)(desc_addr + desc_offset)),
-			cpy_len);
-
-		mbuf_avail  -= cpy_len;
-		mbuf_offset += cpy_len;
-		desc_avail  -= cpy_len;
-		desc_offset += cpy_len;
 	}
 
 	prev->data_len = mbuf_offset;
-- 
1.9.0