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

[Jason Wang]

On 2016å11æ10æ 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 <jasowang@xxxxxxxxxx>
> 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.

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

Ok.

> >   
> > --
> > 2.7.4