[PATCH v2] vhost: Only access header if offloading is supported in dequeue path

[Maxime Coquelin <maxime.coquelin@redhat.com>]

If offloading features are not negotiated, parsing the virtio header
is not needed.

Micro-benchmark with testpmd shows that the gain is +4% with indirect
descriptors, +1% when using direct descriptors.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
Changes since v1:
=================
 - Rebased
 - Fix early out check in vhost_dequeue_offload

 lib/librte_vhost/virtio_net.c | 47 ++++++++++++++++++++++++++++++++-----------
 1 file changed, 35 insertions(+), 12 deletions(-)

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index a59c39b..5e2fd75 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -548,6 +548,18 @@ rte_vhost_enqueue_burst(int vid, uint16_t queue_id,
 		return virtio_dev_rx(dev, queue_id, pkts, count);
 }
 
+static inline bool
+virtio_net_with_host_offload(struct virtio_net *dev)
+{
+	if (dev->features &
+			(VIRTIO_NET_F_CSUM | VIRTIO_NET_F_HOST_ECN |
+			 VIRTIO_NET_F_HOST_TSO4 | VIRTIO_NET_F_HOST_TSO6 |
+			 VIRTIO_NET_F_HOST_UFO))
+		return true;
+
+	return false;
+}
+
 static void
 parse_ethernet(struct rte_mbuf *m, uint16_t *l4_proto, void **l4_hdr)
 {
@@ -600,6 +612,9 @@ vhost_dequeue_offload(struct virtio_net_hdr *hdr, struct rte_mbuf *m)
 	void *l4_hdr = NULL;
 	struct tcp_hdr *tcp_hdr = NULL;
 
+	if (hdr->flags == 0 && hdr->gso_type == VIRTIO_NET_HDR_GSO_NONE)
+		return;
+
 	parse_ethernet(m, &l4_proto, &l4_hdr);
 	if (hdr->flags == VIRTIO_NET_HDR_F_NEEDS_CSUM) {
 		if (hdr->csum_start == (m->l2_len + m->l3_len)) {
@@ -684,12 +699,12 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vring_desc *descs,
 		  struct rte_mempool *mbuf_pool)
 {
 	struct vring_desc *desc;
-	uint64_t desc_addr;
+	uint64_t desc_addr = 0;
 	uint32_t desc_avail, desc_offset;
 	uint32_t mbuf_avail, mbuf_offset;
 	uint32_t cpy_len;
 	struct rte_mbuf *cur = m, *prev = m;
-	struct virtio_net_hdr *hdr;
+	struct virtio_net_hdr *hdr = NULL;
 	/* A counter to avoid desc dead loop chain */
 	uint32_t nr_desc = 1;
 
@@ -698,12 +713,14 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vring_desc *descs,
 			(desc->flags & VRING_DESC_F_INDIRECT))
 		return -1;
 
-	desc_addr = gpa_to_vva(dev, desc->addr);
-	if (unlikely(!desc_addr))
-		return -1;
+	if (virtio_net_with_host_offload(dev)) {
+		desc_addr = gpa_to_vva(dev, desc->addr);
+		if (unlikely(!desc_addr))
+			return -1;
 
-	hdr = (struct virtio_net_hdr *)((uintptr_t)desc_addr);
-	rte_prefetch0(hdr);
+		hdr = (struct virtio_net_hdr *)((uintptr_t)desc_addr);
+		rte_prefetch0(hdr);
+	}
 
 	/*
 	 * A virtio driver normally uses at least 2 desc buffers
@@ -720,18 +737,24 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vring_desc *descs,
 		if (unlikely(!desc_addr))
 			return -1;
 
-		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 {
+		if (!desc_addr) {
+			desc_addr = gpa_to_vva(dev, desc->addr);
+			if (unlikely(!desc_addr))
+				return -1;
+		}
+
 		desc_avail  = desc->len - dev->vhost_hlen;
 		desc_offset = dev->vhost_hlen;
 	}
 
+	rte_prefetch0((void *)(uintptr_t)(desc_addr + desc_offset));
+
+	PRINT_PACKET(dev, (uintptr_t)(desc_addr + desc_offset), desc_avail, 0);
+
 	mbuf_offset = 0;
 	mbuf_avail  = m->buf_len - RTE_PKTMBUF_HEADROOM;
 	while (1) {
@@ -795,7 +818,7 @@ copy_desc_to_mbuf(struct virtio_net *dev, struct vring_desc *descs,
 	prev->data_len = mbuf_offset;
 	m->pkt_len    += mbuf_offset;
 
-	if (hdr->flags != 0 || hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE)
+	if (virtio_net_with_host_offload(dev))
 		vhost_dequeue_offload(hdr, m);
 
 	return 0;
-- 
2.7.4