linux-kernel - [PATCH] ringtest: Look at next descriptor before setting up event index

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Date:   Mon, 19 Sep 2016 13:38:59 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     mst@...hat.com
Subject: [PATCH] ringtest: Look at next descriptor before setting up event index

This avoids an expensive memory barrier if the next descriptor
is already there.  The benefit is around 10%.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 tools/virtio/ringtest/ring.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/tools/virtio/ringtest/ring.c b/tools/virtio/ringtest/ring.c
index c25c8d248b6b..e617e20538bf 100644
--- a/tools/virtio/ringtest/ring.c
+++ b/tools/virtio/ringtest/ring.c
@@ -182,6 +182,9 @@ bool enable_call()
 {
 	unsigned head = (ring_size - 1) & guest.last_used_idx;
 
+	if (!(ring[head].flags & DESC_HW))
+		return false;
+
 	event->call_index = guest.last_used_idx;
 	/* Flush call index write */
 	/* Barrier D (for pairing) */
@@ -215,6 +218,9 @@ bool enable_kick()
 {
 	unsigned head = (ring_size - 1) & host.used_idx;
 
+	if (ring[head].flags & DESC_HW)
+		return false;
+
 	event->kick_index = host.used_idx;
 	/* Barrier C (for pairing) */
 	smp_mb();
-- 
2.7.4