Re: [Xen-devel] reboot driver domain, vifX.Y = NO-CARRIER?

[Jason Cooper <xen@xxxxxxxxxxxxxx>]

On Mon, Apr 30, 2018 at 05:26:38PM +0100, Ian Jackson wrote:
> Jason Cooper writes ("Re: [Xen-devel] reboot driver domain, vifX.Y = 
> NO-CARRIER?"):
> > On Mon, Apr 30, 2018 at 04:22:30PM +0100, Ian Jackson wrote:
...
> > Ok, so I'm new to the guts of Xen.  The bug, at a high level, is that
> > "When a driver domain is rebooted (domid changed), previously connected
> > client domUs can't gain network connectivity to/through the driver
> > domain via 'xl network-attach client_domu mac=... bridge=...
> > backend=drv_dom'"
> > 
> > This is due to the fact that the frontend net driver doesn't / can't
> > follow the backend driver to the new domid in xenstore.
> 
> Yes.
> 
> > > I'm a bit surprised that this doesn't already work.
> > 
> > I'm currently running Xen 4.9.1 as patched in the standard Gentoo
> > ebuild.  I've been putting off upgrading to 4.9.2, now marked stable in
> > portage, until I nail this down.  I'm happy to move to 4.10 if needed.
> > 
> > Do you think this is something that is definitely fixed in a more recent
> > version of Xen?  I'm happy to test if so.  Is there a commit id I can
> > look for?
> 
> I think that in my view (which others may disagree with) this is not a
> bug in Xen but in the Linux kernel frontend.  So changing the Xen
> version won't help.

I'm running vanilla v4.16.4 based on allnoconfig in all of these
mini-domu's.  It doesn't look there's been any pertinent recent changes
in drivers/net/xen-netfront.c since v4.16.

Based on an initial scan of the code, it looks like xen-netback watches
for hotplug events on the frontend (xen-netback/xenbus.c:1041-1046 in
connect()).  xen-netfront.c:1995-2036, netback_changed(), is the
registered callback for netfront.

Is the xenbus netback/netfront state machine documented anywhere?
include/xen/interface/io/netif.h has a great description of tx/rx queue
setup and teardown, but doesn't seem to have anything specific to the
high-level signalling that 'xl network-attach' would cause.

Any pointers?

thx,

Jason.

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