[libvirt] inconsistent handling of "qemu64" CPU model

[Chris Friesen]

Hi,

I'm not sure where the problem lies, hence the CC to both lists.  Please copy me 
on the reply.

I'm playing with OpenStack's devstack environment on an Ubuntu 14.04 host with a 
Celeron 2961Y CPU.  (libvirt detects it as a Nehalem with a bunch of extra 
features.)  Qemu gives version 2.2.0 (Debian 1:2.2+dfsg-5expubuntu9.7~cloud2).

If I don't specify a virtual CPU model, it appears to give me a "qemu64" CPU, 
and /proc/cpuinfo in the guest instance looks something like this:

processor 0
vendor_id GenuineIntel
cpu family 6
model 6
model name: QEMU Virtual CPU version 2.2.0
stepping: 3
microcode: 0x1
flags: fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pse36 clflush
mmx fxsr sse sse2 syscall nx lm rep_good nopl pni vmx cx16 x2apic popcnt 
hypervisor lahf_lm abm vnmi ept


However, if I explicitly specify a custom CPU model of "qemu64" the instance 
refuses to boot and I get a log saying:

libvirtError: unsupported configuration: guest and host CPU are not compatible: 
Host CPU does not provide required features: svmlibvirtError: unsupported 
configuration: guest and host CPU are not compatible: Host CPU does not provide 
required features: svm

When this happens, some of the XML for the domain looks like this:
   <os>
     <type arch='x86_64' machine='pc-i440fx-utopic'>hvm</type>
  ....

   <cpu mode='custom' match='exact'>
     <model fallback='allow'>qemu64</model>
     <topology sockets='1' cores='1' threads='1'/>
   </cpu>

Of course "svm" is an AMD flag and I'm running an Intel CPU.  But why does it 
work when I just rely on the default virtual CPU?  Is kvm_default_unset_features 
handled differently when it's implicit vs explicit?

If I explicitly specify a custom CPU model of "kvm64" then it boots, but of 
course I get a different virtual CPU from what I get if I don't specify anything.

Following some old suggestions I tried turning off nested kvm, deleting 
/var/cache/libvirt/qemu/capabilities/*, and restarting libvirtd.  Didn't help.

So...anyone got any ideas what's going on?  Is there no way to explicitly 
specify the model that you get by default?


Thanks,
Chris