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

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Tue, May 18, 2021 at 09:48:25AM +0000, Durrant, Paul wrote:
> > -----Original Message-----
> > From: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> > 
> > On Tue, May 18, 2021 at 09:34:45AM +0000, Durrant, Paul wrote:
> > > > -----Original Message-----
> > > > From: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> > > >
> > > > 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

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