From f3bf9a1a9b1ad3419b436855306ad8b5d8efab2f Mon Sep 17 00:00:00 2001

From: Maxime Coquelin <maxime.coquelin@redhat.com>

Date: Thu, 20 Dec 2018 17:47:55 +0100

Subject: [PATCH 18/18] vhost: batch used descs chains write-back with packed

 ring

[ upstream commit b473ec1131ee44ee25e0536a04be65246b93f4f3 ]

Instead of writing back descriptors chains in order, let's

write the first chain flags last in order to improve batching.

Also, move the write barrier in logging cache sync, so that it

is done only when logging is enabled. It means there is now

one more barrier for split ring when logging is enabled.

With Kernel's pktgen benchmark, ~3% performance gain is measured.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>

(cherry picked from commit b473ec1131ee44ee25e0536a04be65246b93f4f3)

Signed-off-by: Jens Freimann <jfreimann@redhat.com>

---

 lib/librte_vhost/vhost.h      |  7 ++-----

 lib/librte_vhost/virtio_net.c | 19 ++++++++++++++++---

 2 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h

index 552b9298d..adc2fb78e 100644

--- a/lib/librte_vhost/vhost.h

+++ b/lib/librte_vhost/vhost.h

@@ -456,12 +456,9 @@ vhost_log_cache_sync(struct virtio_net *dev, struct vhost_virtqueue *vq)

 		   !dev->log_base))

 		return;

-	log_base = (unsigned long *)(uintptr_t)dev->log_base;

+	rte_smp_wmb();

-	/*

-	 * It is expected a write memory barrier has been issued

-	 * before this function is called.

-	 */

+	log_base = (unsigned long *)(uintptr_t)dev->log_base;

 	for (i = 0; i < vq->log_cache_nb_elem; i++) {

 		struct log_cache_entry *elem = vq->log_cache + i;

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c

index 15d682c3c..ec70ef947 100644

--- a/lib/librte_vhost/virtio_net.c

+++ b/lib/librte_vhost/virtio_net.c

@@ -136,6 +136,8 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,

 {

 	int i;

 	uint16_t used_idx = vq->last_used_idx;

+	uint16_t head_idx = vq->last_used_idx;

+	uint16_t head_flags = 0;

 	/* Split loop in two to save memory barriers */

 	for (i = 0; i < vq->shadow_used_idx; i++) {

@@ -165,12 +167,17 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,

 			flags &= ~VRING_DESC_F_AVAIL;

 		}

-		vq->desc_packed[vq->last_used_idx].flags = flags;

+		if (i > 0) {

+			vq->desc_packed[vq->last_used_idx].flags = flags;

-		vhost_log_cache_used_vring(dev, vq,

+			vhost_log_cache_used_vring(dev, vq,

 					vq->last_used_idx *

 					sizeof(struct vring_packed_desc),

 					sizeof(struct vring_packed_desc));

+		} else {

+			head_idx = vq->last_used_idx;

+			head_flags = flags;

+		}

 		vq->last_used_idx += vq->shadow_used_packed[i].count;

 		if (vq->last_used_idx >= vq->size) {

@@ -179,7 +186,13 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,

 		}

 	}

-	rte_smp_wmb();

+	vq->desc_packed[head_idx].flags = head_flags;

+

+	vhost_log_cache_used_vring(dev, vq,

+				head_idx *

+				sizeof(struct vring_packed_desc),

+				sizeof(struct vring_packed_desc));

+

 	vq->shadow_used_idx = 0;

 	vhost_log_cache_sync(dev, vq);

 }

-- 

2.21.0