vhost: batch used descs chains write-back with packed ring

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>

diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
index 3b3265c..f267f89 100644 (file)
--- a/lib/librte_vhost/vhost.h
+++ b/lib/librte_vhost/vhost.h
@@ -457,12 +457,9 @@ 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 4d201a3..474acf6 100644 (file)
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -97,6 +97,8 @@ 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++) {
@@ -126,12 +128,17 @@ 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) {
@@ -140,7 +147,13 @@ 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);
 }