RE: [PATCH v2 4/4] tools/hotplug: modify set_mtu() to inform the frontend via xenstore

[Ian Jackson <ian.jackson@xxxxxxxxxx>]

Paul Durrant writes ("RE: [PATCH v2 4/4] tools/hotplug: modify set_mtu() to 
inform the frontend via xenstore"):
> > -----Original Message-----
> > From: Ian Jackson <ian.jackson@xxxxxxxxxx>
...
> > Actually.
> > 
> > This shouldn't be in the frontend at all, should it ?  In general the
> > backend writes to the backend and the frontend to the frontend.
> > 
> > So maybe I need to take back my R-b of
> >   [PATCH v2 3/4] public/io/netif: specify MTU override node
> > 
> > Sorry for the confusion.  I seem rather undercaffienated today.
> 
> Too late. The xenstore node has been used by Windows frontends for the best 
> part of a decade so we can't practically change the
> path. Another way would be to also modify netback to simply echo the value 
> from backend into frontend, but that seems rather
> pointless.

Hmm.  How does this interact with driver domains ?  I think a driver
domain might not have write access to this node.

Is there a value we can store in it that won't break these Windows
frontends, that libxl in the toolstack domain could write, before the
hotplug script runs in the driver domain ?

> Interestingly libxl does define an 'mtu' field for libxl_device_nic, which it 
> sets to 1492 in libxl__device_nic_setdefault() but
> never writes it into xenstore. There is even a comment:
> 
> /* nic->mtu = */
> 
> in libxl__nic_from_xenstore() which implies it should have been there, but 
> isn't.
> I still think picking up the MTU from the bridge is the better way though. 

I agree that the default should come from the bridge.  Ideally there
would be a way to override it in the config.

Thanks,
Ian.