Re: [Qemu-devel] [RFC 25/29] vhu: enable = false on get_vring_base

[Michael S. Tsirkin]

On Wed, Jul 05, 2017 at 06:16:17PM +0100, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (address@hidden) wrote:
> > On Wed, Jun 28, 2017 at 08:00:43PM +0100, Dr. David Alan Gilbert (git) 
> > wrote:
> > > From: "Dr. David Alan Gilbert" <address@hidden>
> > > 
> > > When we receive a GET_VRING_BASE message set enable = false
> > > to stop any new received packets modifying the ring.
> > > 
> > > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > 
> > I think I already reviewed a similar patch.
> 
> Yes you replied to my off-list mail; I hadn't got
> around to fixing it yet.
> 
> > Spec says:
> > Client must only process each ring when it is started.
> 
> but in that reply you said the spec said 
> 
>   Client must only pass data between the ring and the
>   backend, when the ring is enabled.
> 
> So does the spec say 'started' or 'enabled'

Both. Ring processing is limited by ring being started.
Passing actual data - by ring also being enabled.
With ring started but disabled you drop all packets.

> (Pointer to the spec?)

It's part of QEMU source:
docs/interop/vhost-user.txt


> > IMHO the real fix is to fix client to check the started
> > flag before processing the ring.
> 
> Yep I can do that.  I was curious however whether it was
> specified as 'started' or 'enabled' or both.
> 
> Dave
> 
> > > ---
> > >  contrib/libvhost-user/libvhost-user.c | 1 +
> > >  1 file changed, 1 insertion(+)
> > > 
> > > diff --git a/contrib/libvhost-user/libvhost-user.c 
> > > b/contrib/libvhost-user/libvhost-user.c
> > > index ceddeac74f..d37052b7b0 100644
> > > --- a/contrib/libvhost-user/libvhost-user.c
> > > +++ b/contrib/libvhost-user/libvhost-user.c
> > > @@ -652,6 +652,7 @@ vu_get_vring_base_exec(VuDev *dev, VhostUserMsg *vmsg)
> > >      vmsg->size = sizeof(vmsg->payload.state);
> > >  
> > >      dev->vq[index].started = false;
> > > +    dev->vq[index].enable = false;
> > >      if (dev->iface->queue_set_started) {
> > >          dev->iface->queue_set_started(dev, index, false);
> > >      }
> > > -- 
> > > 2.13.0
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK