Description of problem:

1) Prior to libvirt-0.9.10, libvirt would implicitly add a usb controller to the qemu commandline. Before libvirt-0.9.5 libvirt didn't support explicit specification of the USB controller in the domain's XML.

2) Starting with libvirt-0.9.5, libvirt recognized definition of a USB controller, e.g.:

  <controller type='usb' index='0'/>

(this was added as part of USB 2.0 support).

3) Starting with libvirt-0.9.10, any time a domain configuration is read by libvirt, the following line is added to the devices section:

  <controller type='usb' index='0'/>

4) RHEL6.2 libvirt is based on upstream 0.9.4, RHEL6.3 libvirt is based on 0.9.10.

The problem is that when a guest is started on a host running libvirt >= 0.9.10 (e.g. RHEL6.3), and then later migrated to a different host that is running libvirt < 0.9.5 (e.g. RHEL6.2), this migration will *always* fail with:

   error: internal error Unknown controller type 'usb'

(Note that the guest in question is otherwise compatible, e.g. it was created on a RHEL6.2 host and has machine type "rhel6.2.0".)

Version-Release number of selected component (if applicable): source libvirt-0.9.10-1 or later, destination libvirt-0.9.4-anything or earlier

How reproducible: 100%


Steps to Reproduce:
1. define and install a guest on a RHEL6.2 host
2. verify that it migrates to and from another RHEL6.2 host.
3. upgrade one host to RHEL6.3

4. migrate the guest from the RHEL6.2 host to RHEL6.3 (works)
5. attempt to migrate the guest from the RHEL6.3 host to RHEL6.2 (fails)
  
Actual results:

  Step 5 gives:

   error: internal error Unknown controller type 'usb'

Expected results:

   migration should be successful in both directions

Additional info:

An example of this failure is documented in https://bugzilla.redhat.com/show_bug.cgi?id=806633#c29

AFAIK, RHEL guarantees that migration back and forth between hosts running different minor releases of RHEL will work (as long as the config of the guest is unchanged). This bug breaks that guarantee.