We do not need the most up to date number of heads, we only want to
know if there is at least one.
Use shadow variable as long as it is not equal to the last available
index checked. This avoids expensive qatomic dereference of the
RCU-protected memory region cache as well as the memory access itself
and the subsequent memory barrier.
The change improves performance of the af-xdp network backend by 2-3%.
Signed-off-by: Ilya Maximets <i.maxim...@ovn.org>
---
hw/virtio/virtio.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 309038fd46..04bf7cc977 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -999,7 +999,15 @@ void virtqueue_push(VirtQueue *vq, const VirtQueueElement
*elem,
/* Called within rcu_read_lock(). */
static int virtqueue_num_heads(VirtQueue *vq, unsigned int idx)
{
- uint16_t num_heads = vring_avail_idx(vq) - idx;
+ uint16_t num_heads;
+
+ if (vq->shadow_avail_idx != idx) {
+ num_heads = vq->shadow_avail_idx - idx;
+
+ return num_heads;
+ }
+
+ num_heads = vring_avail_idx(vq) - idx;
/* Check it isn't doing very strange things with descriptor numbers. */
if (num_heads > vq->vring.num) {
--
2.40.1
