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vhost: better detection of available buffers

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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>
This commit is contained in:
Jason Wang 2017-01-18 15:02:01 +08:00 committed by David S. Miller
parent 1a8b6d76dc
commit 275bf960ac
1 changed files with 7 additions and 3 deletions
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10
drivers/vhost/vhost.c
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@ -2241,11 +2241,15 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq)
__virtio16 avail_idx;
int r;
r = vhost_get_user(vq, avail_idx, &vq->avail->idx);
if (r)
if (vq->avail_idx != vq->last_avail_idx)
return false;
return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx;
r = vhost_get_user(vq, avail_idx, &vq->avail->idx);
if (unlikely(r))
return false;
vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
return vq->avail_idx == vq->last_avail_idx;
}
EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);