Re: About the kvm-no-adjvtime CPU property

[Ying Fang]

On 2020/6/1 16:07, Andrew Jones wrote:
> 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
yes，the cpu model is indeed used with kvm.

There is a situation where the host cpu model is Cortex-A72 and
a 'custom' cpu mode is used to keep insrtuction set compatible between
the source and destination host machine when doing live migration.
So the host physical machine cpu model is Cortex-A72 but
host-passthrough model is mode used here.

> 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.
In the above scenario, the cpu mode is Cortex-A72 but maybe only a 
subset of cpu feature is presented to the guest.

Since the cpu model is not 'host' here, the kvm-adjvtime is not enabled 
by default. The guest virtual time will not be adjusted.

> Thanks,
> drew
>
>
> .