[dpdk-dev] [PATCH v2] vhost: optimize broadcast rarp sync with c11 atomic

[Phil Yang <phil.yang@arm.com>]

The rarp packet broadcast flag is synchronized with rte_atomic_XX APIs
which is a full barrier, DMB, on aarch64. This patch optimized it with
c11 atomic one-way barrier.

Signed-off-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Joyce Kong <joyce.kong@arm.com>
---
v2:
split from the 'generic rte atomic APIs deprecate proposal' patchset.

 lib/librte_vhost/vhost.h      |  2 +-
 lib/librte_vhost/vhost_user.c |  7 +++----
 lib/librte_vhost/virtio_net.c | 16 +++++++++-------
 3 files changed, 13 insertions(+), 12 deletions(-)

diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
index 2087d14..0e22125 100644
--- a/lib/librte_vhost/vhost.h
+++ b/lib/librte_vhost/vhost.h
@@ -350,7 +350,7 @@ struct virtio_net {
 	uint32_t		flags;
 	uint16_t		vhost_hlen;
 	/* to tell if we need broadcast rarp packet */
-	rte_atomic16_t		broadcast_rarp;
+	int16_t			broadcast_rarp;
 	uint32_t		nr_vring;
 	int			dequeue_zero_copy;
 	int			extbuf;
diff --git a/lib/librte_vhost/vhost_user.c b/lib/librte_vhost/vhost_user.c
index bd1be01..857187d 100644
--- a/lib/librte_vhost/vhost_user.c
+++ b/lib/librte_vhost/vhost_user.c
@@ -2145,11 +2145,10 @@ vhost_user_send_rarp(struct virtio_net **pdev, struct VhostUserMsg *msg,
 	 * Set the flag to inject a RARP broadcast packet at
 	 * rte_vhost_dequeue_burst().
 	 *
-	 * rte_smp_wmb() is for making sure the mac is copied
-	 * before the flag is set.
+	 * __ATOMIC_RELEASE ordering is for making sure the mac is
+	 * copied before the flag is set.
 	 */
-	rte_smp_wmb();
-	rte_atomic16_set(&dev->broadcast_rarp, 1);
+	__atomic_store_n(&dev->broadcast_rarp, 1, __ATOMIC_RELEASE);
 	did = dev->vdpa_dev_id;
 	vdpa_dev = rte_vdpa_get_device(did);
 	if (vdpa_dev && vdpa_dev->ops->migration_done)
diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 37c47c7..fa10deb 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -2203,6 +2203,7 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
 	struct virtio_net *dev;
 	struct rte_mbuf *rarp_mbuf = NULL;
 	struct vhost_virtqueue *vq;
+	int16_t success = 1;
 
 	dev = get_device(vid);
 	if (!dev)
@@ -2249,16 +2250,17 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
 	 *
 	 * broadcast_rarp shares a cacheline in the virtio_net structure
 	 * with some fields that are accessed during enqueue and
-	 * rte_atomic16_cmpset() causes a write if using cmpxchg. This could
-	 * result in false sharing between enqueue and dequeue.
+	 * __atomic_compare_exchange_n causes a write if performed compare
+	 * and exchange. This could result in false sharing between enqueue
+	 * and dequeue.
 	 *
 	 * Prevent unnecessary false sharing by reading broadcast_rarp first
-	 * and only performing cmpset if the read indicates it is likely to
-	 * be set.
+	 * and only performing compare and exchange if the read indicates it
+	 * is likely to be set.
 	 */
-	if (unlikely(rte_atomic16_read(&dev->broadcast_rarp) &&
-			rte_atomic16_cmpset((volatile uint16_t *)
-				&dev->broadcast_rarp.cnt, 1, 0))) {
+	if (unlikely(__atomic_load_n(&dev->broadcast_rarp, __ATOMIC_ACQUIRE) &&
+			__atomic_compare_exchange_n(&dev->broadcast_rarp,
+			&success, 0, 0, __ATOMIC_RELEASE, __ATOMIC_RELAXED))) {
 
 		rarp_mbuf = rte_net_make_rarp_packet(mbuf_pool, &dev->mac);
 		if (rarp_mbuf == NULL) {
-- 
2.7.4