This avoids an expensive memory barrier if the next descriptor
is already there. The benefit is around 10%.
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
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
--
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