Re: About the kvm-no-adjvtime CPU property

[Ying Fang]

On 2020/6/1 20:29, Andrew Jones wrote:
> On Mon, Jun 01, 2020 at 08:07:31PM +0800, Ying Fang wrote:
> > 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.
I mean host-passthrough model is 'not' used here. Sorry to make it 
confusing.
> Are the source and destinations hosts used in the migration identical?
> If so, then the guest can use cpu 'host' and disable cpu features that
> should not be exposed (e.g. -cpu host,pmu=off).  If the source and
> destination hosts are not identical, then I'm curious what those exact
> differences are.  With the way AArch64 KVM works today, even using the
> Cortex-A72 cpu model should require identical hosts when migrating.  Or,
> at least both hosts must be compatible with Cortex-A72 and any difference
> in ID registers must be somehow getting hidden from the guest.
Yes, you are right.
We have AAarch64 server with cpu based on Cortex-A72 and some extra 
instruction set added. Source host with cpu based on V1 and destination 
host with cpu based on V2 and they are compatible with Cortex-A72. We 
want to use a 'custom' cpu mode here to make it possible to do live 
migration between them. This is the scenario where the 'host' cpu model 
is not used since a 'custom' cpu model Cortex-A72 is used here . However 
the kvm-adjvtime feature is also need. So I think we should move 
kvm_arm_add_vcpu_properties to arm_cpu_post_init, instead of limited to 
'host' and 'max' cpu model[1].

1: https://lists.gnu.org/archive/html/qemu-devel/2020-06/msg00091.html
> Thanks,
> drew