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In nova/virt/libvirt/driver.py the _compare_cpu() function checks the 
cpu features using self._conn.compareCPU().

Is this actually valid?  The kvm processes don't seem to have the 
"--cpu" option specified, so we should get a compatible subset of cpu 
features from qemu.  If that's the case then the host cpu features 
shouldn't really matter, right?

We're running Havana and hitting live-migration issues between Sandy 
Bridge and Ivy Bridge due to some differences in the host processor 
flags.  Just wondering what the expected behaviour is.

Chris