[libvirt] Incompatible CPU after upgrading qemu

[Jim Fehlig]

Hi All,

I've been doing some upgrade testing and noticed a problem starting some
existing VMs after upgrading qemu from 2.6 to 2.9 on one of my AMD machines.
Using libvirt 3.2.0 and qemu 2.6, I have no problems starting a VM with the
following machine/CPU config

  <os>
    <type arch='x86_64' machine='pc-i440fx-2.6'>hvm</type>
    <boot dev='hd'/>
  </os>
   <cpu mode='custom' match='exact'>
    <model fallback='allow'>Opteron_G4</model>
  </cpu>

After upgrading qemu to 2.9 the VM fails to start with

error: the CPU is incompatible with host CPU: Host CPU does not provide required
features: svm

I see that qemu commit 75d373ef disables SVM in KVM mode, but that change was
made quite some time ago and pertains to all machine types > pc-i440fx-2.2.
Perhaps the problem was revealed by Jiri's recent libvirt changes to detect host
CPU model by asking qemu?

My knee-jerk reaction is to remove 'svm' from Opteron CPUs in cpu_map.xml, but
I'm far from the expert on the evolving CPU code in libvirt, so asking the
experts here for some guidance.

Regards,
Jim