Commit 275bf960 authored Jan 18, 2017 by Jason Wang Committed by David S. Miller Jan 18, 2017

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>

parent 1a8b6d76

drivers/vhost/vhost.c

+6 −2

Original line number	Original line	Diff line number	Diff line
	@@ -2241,11 +2241,15 @@ bool vhost_vq_avail_empty(struct vhost_dev dev, struct vhost_virtqueue vq)
	__virtio16 avail_idx;		__virtio16 avail_idx;
	int r;		int r;

			if (vq->avail_idx != vq->last_avail_idx)
			return false;

	r = vhost_get_user(vq, avail_idx, &vq->avail->idx);		r = vhost_get_user(vq, avail_idx, &vq->avail->idx);
	if (r)		if (unlikely(r))
	return false;		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);		EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);