[libvirt-users] Usable and non-usable CPU models in nested virtualization

[Milan Zamazal]

Hi, some custom CPU models are reported from
virConnectGetDomainCapabilities as usable='yes' on a physical machine
while as usable='no' inside a VM running on the same machine.  That's
not completely surprising.

But what surprises me is that those models are still reported from
virConnectCompareCPU as supported (VIR_CPU_COMPARE_SUPERSET) in the
nested environment and VMs can be started happily with them.

For instance, virConnectGetDomainCapabilities reports

  <model usable='no'>Skylake-Client</model>

but when I try to use that model anyway, the VM starts fine with it:

  <cpu mode='custom' match='exact' check='full'>
    <model fallback='forbid'>Skylake-Client</model>
    <topology sockets='16' cores='1' threads='1'/>
    <feature policy='require' name='hypervisor'/>
    <feature policy='disable' name='invpcid'/>
    <numa>
      <cell id='0' cpus='0' memory='524288' unit='KiB'/>
    </numa>
  </cpu>

That's actually good news, but unexpected.  Do I miss something?

Thanks,
Milan