Xen 4.x / Linux 3.x (dom0 and HVM domU) and NIC handling

[Stefan Bader <stefan.bader@canonical.com>]

Moving to public discussion...

This was found with Xen hypervisor version supporting device unplugging and the
domU kernel having net-/blkfront and pci platform built-in (or as module).

The block device is defined as hda and the NIC type=ioemu (so theoretically
guests without pv support would work, too).

Since both drivers are present, the kernel tries to unplug the emulated devices
and succeeds. The blkfront driver detects the xvda device available in parallel
and is working ok.

However the network interface does not work. There are entries present under
sysfs for the xenbus but trying to bring it up fails with errors. And also there
seems to be no mac address set (all zeros in sysfs).
When the type=ioemu is removed in the configuration, this works.

I have not much more debugging information beyond that, yet. But it sounds a bit
like NICs should behave the same as block devices. So if there is an emulated
device defined there will be an alternate paravirt interface for it and after
unplugging the emulated ones we end up with the pv ones.
Is that something that can be seen with newer Xen versions, too (I am using 4.1.1)?

-Stefan