Subject: Re: [PATCH] xen-netback: Check for hotplug-status existence before watching

From: Marek Marczykowski-Górecki <[email-masked]>
Date: Tue, 18 May 2021 12:42:09 +0200

On Tue, May 18, 2021 at 09:48:25AM +0000, Durrant, Paul wrote:
> > -----Original Message-----
> > From: Marek Marczykowski-Górecki <[email-masked]>
> > 
> > On Tue, May 18, 2021 at 09:34:45AM +0000, Durrant, Paul wrote:
> > > > -----Original Message-----
> > > > From: Marek Marczykowski-Górecki <[email-masked]>
> > > >
> > > > On Tue, May 18, 2021 at 07:57:16AM +0100, Paul Durrant wrote:
> > > > > Why is that missing? We're going behind the back of the toolstack to do the
> > > > > unbind and bind so why should we expect it to re-execute a hotplug script?
> > > >
> > > > Ok, then simply execute the whole hotplug script (instead of its subset)
> > > > after re-loading the backend module and everything will be fine.
> > > >
> > > > For example like this:
> > > >     XENBUS_PATH=backend/vif/$DOMID/$VIF \
> > > >     XENBUS_TYPE=vif \
> > > >     XENBUS_BASE_PATH=backend \
> > > >     script=/etc/xen/scripts/vif-bridge \
> > > >     vif=vif.$DOMID.$VIF \
> > > >     /etc/xen/scripts/vif-bridge online
> > > >
> > >
> > > ... as long as there's no xenstore fall-out that the guest can observe.
> > 
> > Backend will set state to XenbusStateInitWait on load anyway...
> > 
> 
> Oh, that sounds like a bug then... It ought to go straight to connected if the frontend is already there.

To me this sounds very suspicious. But if that's really what should
backend do, then it would "solve" also hotplug-status node issue.
See the end of netback_probe() function.
But I think if you start processing traffic before hotplug script
configures the interface (so - without switching to XenbusStateInitWait
and waiting for hotplug-status node), you'll post some packets into not
enabled interface, which I think will drop them (not queue). TCP will be
fine with that, but many other protocols not.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab