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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 11 Nov 2016 05:41:43 +0200

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.

> > 
> > Maybe we should just rename this to vhost_vq_avail_unchanged
> > to clarify usage.
> > 
> 
> Ok.
> 
> > > -- 
> > > 2.7.4