On Tue, Nov 15, 2016 at 04:00:21PM +0800, Jason Wang wrote: > > > On 2016年11月15日 11:28, Michael S. Tsirkin wrote: > > On Tue, Nov 15, 2016 at 11:16:59AM +0800, Jason Wang wrote: > > > > > > On 2016年11月12日 00:20, Michael S. Tsirkin wrote: > > > > 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. > > > > > > > But as you said, this does not mean (in mergeable cases) we have enough > > > space to store the packet. > > Absolutely but it checks once and then only re-checks after value > > changes again. > > > > Since get_rx_bufs() does not get enough buffers, we will wait for the kick > in this case. For busy polling, we probably want to stay in the busy loop > here.
That's what I'm saying. You don't want to re-poll the queue if available idx was unchanged. -- MST
