[libvirt] Support for kvm=off cpu option

[Cole Robinson]

On 08/11/2014 05:54 PM, Alex Williamson wrote:
> As of QEMU 2.1 we now have a new -cpu option, kvm=on|off which controls
> whether we expose KVM as the hypervisor via the MSR hypervisor nodes.
> The default is on.  The off state is meant to hide kvm from standard
> detection routines.  This allows us to disable paravirtualization
> detection in the guest and can be used to bypass hypervisor checks in
> certain guest drivers[1].
> 
> How would we like to enable this in libvirt?  One option would be a
> processor feature, ex:
> 
>         <feature policy='disable' name='kvm'/>
> 
> But since this doesn't map to a cpuid feature, I don't know if that
> would be acceptable.
> 
> Another option might be to a domain feature, similar to some of the
> other hyper-v related flags.
> 
>         <domain type='kvm'>
>           <features>
>             <kvm>
>               <kvm state='off'>
>             </kvm>
>             <hyperv>
>               <relaxed state='on'>
>               ...
>             </hyperv>
>           </features>
>           ...
>         </domain>
> 
> Any other suggestions?  Preference?  Thanks,

IMO the latter suggestion makes the most sense.

However <kvm state='off'/> could cause confusion, sounds like it turns off kvm
acceleration. Maybe <signature state=on|off>, or hv_signature, something along
those lines.

- Cole