Re: [PATCH net-next V5 1/3] vhost: better detection of available buffers
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2017-01-18 17:03:08
Also in:
kvm, netdev
On Wed, Jan 18, 2017 at 03:02:01PM +0800, Jason Wang wrote:
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 <redacted>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
quoted hunk ↗ jump to hunk
--- 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