[dpdk-dev] [PATCH v3 5/5] vhost: optimize cache access

[Zhihong Wang]

This patch reorders the code to delay virtio header write to optimize cache
access efficiency for cases where the mrg_rxbuf feature is turned on. It
reduces CPU pipeline stall cycles significantly.

---
Changes in v3:

 1. Remove unnecessary memset which causes frontend stall on SNB & IVB.

 2. Rename variables to follow naming convention.

Signed-off-by: Zhihong Wang <zhihong.wang at intel.com>
---
 lib/librte_vhost/vhost_rxtx.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/lib/librte_vhost/vhost_rxtx.c b/lib/librte_vhost/vhost_rxtx.c
index c4abaf1..e3ba4e0 100644
--- a/lib/librte_vhost/vhost_rxtx.c
+++ b/lib/librte_vhost/vhost_rxtx.c
@@ -154,6 +154,7 @@ enqueue_packet(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	uint32_t mbuf_len = 0;
 	uint32_t mbuf_avail = 0;
 	uint32_t copy_len = 0;
+	uint32_t copy_virtio_hdr = 0;
 	uint32_t extra_buffers = 0;
 
 	/* start with the first mbuf of the packet */
@@ -168,15 +169,16 @@ enqueue_packet(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	if (unlikely(!desc_addr))
 		goto error;
 
-	/* handle virtio header */
+	/*
+	 * handle virtio header, the actual write operation
+	 * is delayed for cache optimization.
+	 */
 	virtio_hdr = (struct virtio_net_hdr_mrg_rxbuf *)(uintptr_t)desc_addr;
-	virtio_enqueue_offload(mbuf, &(virtio_hdr->hdr));
+	copy_virtio_hdr = 1;
 	vhost_log_write(dev, desc->addr, dev->vhost_hlen);
 	desc_offset = dev->vhost_hlen;
 	desc_chain_len = desc_offset;
 	desc_addr += desc_offset;
-	if (is_mrg_rxbuf)
-		virtio_hdr->num_buffers = 1;
 
 	/* start copy from mbuf to desc */
 	while (1) {
@@ -228,8 +230,15 @@ enqueue_packet(struct virtio_net *dev, struct vhost_virtqueue *vq,
 				goto rollback;
 		}
 
-		/* copy mbuf data */
+		/* copy virtio header and mbuf data */
 		copy_len = RTE_MIN(desc->len - desc_offset, mbuf_avail);
+		if (copy_virtio_hdr) {
+			copy_virtio_hdr = 0;
+			virtio_enqueue_offload(mbuf, &(virtio_hdr->hdr));
+			if (is_mrg_rxbuf)
+				virtio_hdr->num_buffers = extra_buffers + 1;
+		}
+
 		rte_memcpy((void *)(uintptr_t)desc_addr,
 				rte_pktmbuf_mtod_offset(mbuf, void *,
 					mbuf_len - mbuf_avail),
-- 
2.7.4