Subject: Re: [PATCH 2/3] vhost: better detection of available buffers

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 11 Nov 2016 18:20:44 +0200

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<[email-masked]>
> > > > > >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