commit e5852d04bf87c02d6d0d8e6d8ded4c33030b9c9e
Author: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Date:   Tue Sep 27 17:51:15 2016 +0800

    xxxx
    
    Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>

diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
index 381dc27..41bfeba 100644
--- a/lib/librte_vhost/vhost.h
+++ b/lib/librte_vhost/vhost.h
@@ -61,6 +61,8 @@ struct buf_vector {
 	uint32_t desc_idx;
 };
 
+#define NR_AVAIL_IDX_PREFETCH	32
+
 /**
  * Structure contains variables relevant to RX/TX virtqueues.
  */
@@ -70,7 +72,7 @@ struct vhost_virtqueue {
 	struct vring_used	*used;
 	uint32_t		size;
 
-	/* Last index used on the available ring */
+	uint16_t		last_avail_idx;
 	uint16_t		last_used_idx;
 #define VIRTIO_INVALID_EVENTFD		(-1)
 #define VIRTIO_UNINITIALIZED_EVENTFD	(-2)
@@ -89,6 +91,9 @@ struct vhost_virtqueue {
 	/* Shadow used ring for performance */
 	struct vring_used_elem	*shadow_used_ring;
 	uint32_t		shadow_used_idx;
+
+	uint16_t		next_avail_idx;
+	uint16_t		avail_idx_buf[NR_AVAIL_IDX_PREFETCH];
 } __rte_cache_aligned;
 
 /* Old kernels have no such macro defined */
diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 11a2c1a..1cc22fc 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -170,6 +170,41 @@ flush_used_ring(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	}
 }
 
+/* Fetch NR_AVAIL_IDX_PREFETCH avail entries at once */
+static void
+prefetch_avail_idx(struct vhost_virtqueue *vq)
+{
+	int i;
+
+	for (i = 0; i < NR_AVAIL_IDX_PREFETCH; i++) {
+		vq->avail_idx_buf[i] = vq->avail->ring[(vq->last_avail_idx + i) &
+					(vq->size - 1)];
+	}
+}
+
+static uint16_t
+next_avail_idx(struct vhost_virtqueue *vq)
+{
+	if (vq->next_avail_idx >= NR_AVAIL_IDX_PREFETCH) {
+		prefetch_avail_idx(vq);
+		vq->next_avail_idx = 0;
+		vq->last_avail_idx += NR_AVAIL_IDX_PREFETCH;
+	}
+
+	return vq->avail_idx_buf[vq->next_avail_idx++];
+}
+
+/*
+ * Just peek, but don't move forward the "next_avail_idx" pointer
+ * The caller also has to make sure the point doesn't go beyond
+ * the array.
+ */
+static uint16_t
+peek_next_avail_idx(struct vhost_virtqueue *vq)
+{
+	return vq->avail_idx_buf[vq->next_avail_idx];
+}
+
 static inline int __attribute__((always_inline))
 enqueue_packet(struct virtio_net *dev, struct vhost_virtqueue *vq,
 		uint16_t avail_idx, struct rte_mbuf *mbuf,
@@ -193,7 +228,7 @@ enqueue_packet(struct virtio_net *dev, struct vhost_virtqueue *vq,
 	mbuf_avail = mbuf_len;
 
 	/* get the current desc */
-	desc_current = vq->avail->ring[(vq->last_used_idx) & (vq->size - 1)];
+	desc_current = next_avail_idx(vq);
 	desc_chain_head = desc_current;
 	desc = &vq->desc[desc_current];
 	desc_addr = gpa_to_vva(dev, desc->addr);
@@ -235,9 +270,7 @@ enqueue_packet(struct virtio_net *dev, struct vhost_virtqueue *vq,
 				if (avail_idx == vq->last_used_idx)
 					goto error;
 
-				desc_current =
-					vq->avail->ring[(vq->last_used_idx) &
-					(vq->size - 1)];
+				desc_current = next_avail_idx(vq);
 				desc_chain_head = desc_current;
 				desc_chain_len = 0;
 			} else
@@ -298,6 +331,7 @@ notify_guest(struct virtio_net *dev, struct vhost_virtqueue *vq)
 		eventfd_write(vq->callfd, (eventfd_t)1);
 }
 
+
 uint16_t
 rte_vhost_enqueue_burst(int vid, uint16_t queue_id,
 	struct rte_mbuf **pkts, uint16_t count)
@@ -331,14 +365,15 @@ rte_vhost_enqueue_burst(int vid, uint16_t queue_id,
 
 	/* start enqueuing packets 1 by 1 */
 	vq->shadow_used_idx = 0;
+	vq->next_avail_idx  = 0;
 	used_idx = vq->last_used_idx & (vq->size - 1);
 	avail_idx = *((volatile uint16_t *)&vq->avail->idx);
 	while (pkt_left && avail_idx != vq->last_used_idx) {
 		/* prefetch the next desc */
-		if (pkt_left > 1 && avail_idx != vq->last_used_idx + 1)
-			rte_prefetch0(&vq->desc[vq->avail->ring[
-					(vq->last_used_idx + 1) &
-					(vq->size - 1)]]);
+		if (pkt_left > 1 &&
+		    vq->next_avail_idx + 1 < NR_AVAIL_IDX_PREFETCH) {
+			rte_prefetch0(&vq->desc[peek_next_avail_idx(vq)]);
+		}
 
 		if (enqueue_packet(dev, vq, avail_idx, pkts[pkt_idx],
 					is_mrg_rxbuf))
@@ -347,6 +382,7 @@ rte_vhost_enqueue_burst(int vid, uint16_t queue_id,
 		pkt_idx++;
 		pkt_left--;
 	}
+	vq->last_avail_idx += vq->next_avail_idx;
 
 	/* batch update used ring for better performance */
 	if (likely(vq->shadow_used_idx > 0))