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Re: [Xen-users] Bridging works fine under KVM but not Xen

On Thu, 2014-01-09 at 14:00 -0600, Glenn E. Bailey III wrote:
> > I take it that you have confirmed (with "brctl show" and "ifconfig -a")
> > that the vif and tap devices are being added to the bridge? Can you post
> > the output of those two commands while things are running please.
> >
> > The other options are firewall rules and sysctl things to do with
> > forwarding and arps etc. Posting the output of the iptables and sysctl
> > commands might set of a light bulb for someone. Likewise the route
> > command (in all cases I mean with whatever option shows all the current
> > settings...)
> >
> > How are the guests getting their IP address? DHCP? Where is the DHCP
> > server? If static then are you sure all the netmasks and subnets etc are
> > correct?
> >
> > Have you checked the in-guest network settings, like gateway and name
> > server? How do they compare to the working KVM guest? Have you also
> > compared all of the host settings (brctl, ifconfig, iptables, sysctl,
> > etc) in the two configurations?
> >
> > Are you using Xen4CentOS or some other Xen packages or perhaps building
> > from source? Have you asked on any CentOS lists (they have a virt one I
> > think) -- there may be folks there who are aware of some quirk.
> Ok,
> I think I've narrowed down the issue a bit. So, it appears that when I
> do a the "xl create" it *does* create the tap interface and adds it to
> the bridge. But, shortly after starting the DomU the interface
> disappears.

This is what would be expected if you had "PVHVM" drivers in your HVM
guest. At start of day the guest has access to both an emulated
(vifX.Y-emu) and a PV (vifX.Y) device, which are basically two halves of
the same thing. As part of the boot process *if* the guest supports and
wants to use the PV path then it will "unplug" the emulated devices (by
issuing some magic I/O writes). This is done for safety since you don't
want to be using the same device via two paths (the need for this is
more obvious for block devices since it would lead to data corruption).

I think now is a good time for you to post your guest dmesg logs. It
will probably also be useful to see the output of "xenstore-ls -fp".

> I did an strace -f against the xl create to try and find out how it's
> creating that tap interface, but it only shows it bringing it down and
> then changing the mac. There were some udev rules in place called
> "xen-backend.rules  xend.rules" but I removed them and still see the
> same results.

xl calls the hotplug scripts manually rather than via udev these days (I
don't recall off hand which version you are running, so I don't know if
that is true for you). Also I expect you'd have to restart udev for any
rules change to take affect.


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