vhost: better detection of available buffers

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@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index d6432603880c1343ea2451eba6df1973e6d61822..9f118388a5b7cc6f70b727f8bafdfadf9a401ac2 100644 (file)
--- 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);