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

On Thu, Jan 9, 2014 at 2:55 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
>> 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).

Ah, that makes sense. So even if qemu has "ifname=vif3.X-emu" in it's
command line it'll still revert back to vif3.X? I went ahead and added
"model=e1000" to the vif and xen_platform_pci=0 to witness the
behaviour. It behaved like expected but still unable to route.

> 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".

xenstore-ls -fp: http://pastebin.com/phJ023vZ
dmesg from guest (back with pvhvm config): http://pastebin.com/5tQ220B2

Note that iptables and SELinux is running on the guest. With them
disabled I get the same results.

>> 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.

I rebooted the host machine after removing the udev rules, it's still
a dev box :-)

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