[Pkg-libvirt-maintainers] Bug#841778: Bug#841778: fails to start qemu VMs when <model fallback='allow'>qemu64</model> is set

[Guido Günther]

On Sun, Oct 23, 2016 at 02:43:16PM +0200, Evgeni Golov wrote:
> Package: libvirt-daemon
> Version: 2.3.0-3
> Severity: important
> 
> Ohai,
> 
> since some time (probably since 2.3.0, but I didn't test exactly), VMs created by vagrant-libvirt fail to start on my laptop with the following error:
> error: the CPU is incompatible with host CPU: Host CPU does not provide required features: svm
> 
> Technically, this is totally true, as I have an Intel CPU, and thus have "only" support for vmx, not svm.
> 
> The VM is defined with the following cpu config:
>   <cpu mode='host-model'>
>     <model fallback='allow'>qemu64</model>
>   </cpu>
> 
> Changing this to
>   <cpu mode='host-model'>
>     <model fallback='allow'/>
>   </cpu>
> or
>   <cpu mode='host-model'>
>     <model fallback='allow'>pentium</model>
>   </cpu>
> makes the VM boot just fine. In both cases the CPU visible inside the VM is a Nehalem/Westmere, which matches the physical i7 in my laptop. But the machine has no vmx flag, as I did not enable nesting for kvm_intel.
> 
> Now the obvious questions are:
> * why does setting fallback to qemu64 fail?
> * why did it work before?
> * is it actually a libvirt bug, or should vagrant-libvirt behave differently?

I'd say vagrant-libvirt should use kvm64 (or better not specify a
fallback cpu at all). See here for some more details:

  https://www.redhat.com/archives/libvir-list/2016-May/msg01940.html

I don't think there's a libvirt bug here.
Cheers,
 -- Guido