On Fri, Nov 11, 2016 at 12:18:50PM +0800, Jason Wang wrote: > > > On 2016年11月11日 11:41, Michael S. Tsirkin wrote: > > On Fri, Nov 11, 2016 at 10:18:37AM +0800, Jason Wang wrote: > > > > > > > > > > > >On 2016年11月10日 03:57, Michael S. Tsirkin wrote: > > > > > >On Wed, Nov 09, 2016 at 03:38:32PM +0800, Jason Wang wrote: > > > > > > > >We should use vq->last_avail_idx instead of vq->avail_idx in the > > > > > > > >checking of vhost_vq_avail_empty() since latter is the cached > > > > > > > >avail > > > > > > > >index from guest but we want to know if there's pending available > > > > > > > >buffers in the virtqueue. > > > > > > > > > > > > > > > >Signed-off-by: Jason Wang<jasow...@redhat.com> > > > > > >I'm not sure why is this patch here. Is it related to > > > > > >batching somehow? > > > > > > > >Yes, we need to know whether or not there's still buffers left in the > > > >virtqueue, so need to check last_avail_idx. Otherwise, we're checking if > > > >guest has submitted new buffers. > > > > > > > > > > > > > > > > > > > > > > > >--- > > > > > > > > drivers/vhost/vhost.c | 2 +- > > > > > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > > > > > > > > >diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > > > > > > > >index c6f2d89..fdf4cdf 100644 > > > > > > > >--- a/drivers/vhost/vhost.c > > > > > > > >+++ b/drivers/vhost/vhost.c > > > > > > > >@@ -2230,7 +2230,7 @@ bool vhost_vq_avail_empty(struct vhost_dev > > > > > > > >*dev, struct vhost_virtqueue *vq) > > > > > > > > if (r) > > > > > > > > return false; > > > > > > > >- return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx; > > > > > > > >+ return vhost16_to_cpu(vq, avail_idx) == > > > > > > > >vq->last_avail_idx; > > > > > > > > } > > > > > > > > EXPORT_SYMBOL_GPL(vhost_vq_avail_empty); > > > > > >That might be OK for TX but it's probably wrong for RX > > > > > >where the fact that used != avail does not mean > > > > > >we have enough space to store the packet. > > > > > > > >Right, but it's no harm since it was just a hint, handle_rx() can handle > > > >this situation. > > Means busy polling will cause useless load on the CPU though. > > > > Right, but,it's not easy to have 100% correct hint here. Needs more thought.
What's wrong with what we have? It polls until value changes. -- MST
