Re: About the kvm-no-adjvtime CPU property

[Andrew Jones]

On Sat, May 30, 2020 at 04:56:26PM +0800, Ying Fang wrote:
> About the kvm-no-adjvtime CPU property
> 
> Hi Andrew,
> To adjust virutal time, a new kvm cpu property kvm-no-adjvtime
> was introduced to 5.0 virt machine types. However the cpu
> property was enabled only for host-passthrough and max cpu model.
> As for other cpu model like cortex-a57, cortex-a53, cortex-a72,
> this kvm-adjvtime is not enabled by default, which means the
> virutal time can not be adjust for them.
> 
> Here, for example, if VM is configured with kvm enabled:
> 
>   <cpu mode='custom' match='exact' check='partial'>
>     <model fallback='allow'>cortex-a72</model>
>     <topology sockets='2' dies='1' cores='2' threads='1'/>
>     <numa>
>       <cell id='0' cpus='0-1' memory='16777216' unit='KiB'/>
>       <cell id='1' cpus='2-3' memory='16777216' unit='KiB'/>
>     </numa>
>   </cpu>
> 
> We cannot adjust virtual time even if 5.0 virt machine is used.
> So i'd like to add it to other cpu model, do you have any
> suggestions here ?
> 
>

Hi Fang,

The cpu feature only requires kvm.  If a cpu model may be used with kvm,
then the feature can be allowed to be used with the model.  What I find
interesting is that the cpu model is being used with kvm instead of 'host'
or 'max'.  Can you explain the reasons for that?  Currently, when using
kvm, the guest will always effectively get 'host' anyway, even when a
model is provided. Indeed, for a model to work, kvm requires it to exactly
match the host cpu and even then the ID registers are passed through from
the host CPU, not taken from QEMU's model.

Thanks,
drew