Re: [PATCH V2 03/11] perf/x86: Add support for TSC in nanoseconds as a perf event clock

[Juergen Gross]

On 07.03.22 11:38, Peter Zijlstra wrote:
> On Mon, Mar 07, 2022 at 11:06:46AM +0100, Juergen Gross wrote:
>
> > > diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
> > > index 4420499f7bb4..a1f179ed39bf 100644
> > > --- a/arch/x86/kernel/paravirt.c
> > > +++ b/arch/x86/kernel/paravirt.c
> > > @@ -145,6 +145,15 @@ DEFINE_STATIC_CALL(pv_sched_clock, native_sched_clock);
> > >    void paravirt_set_sched_clock(u64 (*func)(void))
> > >    {
> > > +	/*
> > > +	 * Anything with ART on promises to have sane TSC, otherwise the whole
> > > +	 * ART thing is useless. In order to make ART useful for guests, we
> > > +	 * should continue to use the TSC. As such, ignore any paravirt
> > > +	 * muckery.
> > > +	 */
> > > +	if (cpu_feature_enabled(X86_FEATURE_ART))
> > > +		return;
> > > +
> > >    	static_call_update(pv_sched_clock, func);
> > >    }
> >
> > NAK, this will break live migration of a guest coming from a host
> > without this feature.
>
> I thought the whole live-migration nonsense made sure to equalize crud
> like that. That is, then don't expose ART to the guest.

Oh, right. I managed to confuse host-side and guest-side usage.

Sorry for the noise.


Juergen

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