[PATCH V4 net-next 1/3] vhost: better detection of available buffers

Jason Wang
Fri, 06 Jan 2017 02:13:15 -0000

This patch tries to do several tweaks on vhost_vq_avail_empty() for a
better performance:

- check cached avail index first which could avoid userspace memory access.
- using unlikely() for the failure of userspace access
- check vq->last_avail_idx instead of cached avail index as the last
  step.

This patch is need for batching supports which needs to peek whether
or not there's still available buffers in the ring.

Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
Signed-off-by: Jason Wang <jasowang at redhat.com>
---
 drivers/vhost/vhost.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index d643260..9f11838 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2241,11 +2241,15 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, struct
vhost_virtqueue *vq)
 	__virtio16 avail_idx;
 	int r;

+	if (vq->avail_idx != vq->last_avail_idx)
+		return false;
+
 	r = vhost_get_user(vq, avail_idx, &vq->avail->idx);
-	if (r)
+	if (unlikely(r))
 		return false;
+	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);

-	return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx;
+	return vq->avail_idx == vq->last_avail_idx;
 }
 EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);

--
2.7.4