Re: [PATCH] perf/x86: Don't enforce minimum period for KVM guest-only events

[Sean Christopherson <seanjc@google.com>]

On Tue, Nov 07, 2023, Mingwei Zhang wrote:
> On Tue, Nov 07, 2023, Sean Christopherson wrote:
> >  arch/x86/events/core.c | 21 +++++++++++++++------
> >  1 file changed, 15 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> > index 40ad1425ffa2..f8a8a4ea4d47 100644
> > --- a/arch/x86/events/core.c
> > +++ b/arch/x86/events/core.c
> > @@ -1388,16 +1388,25 @@ int x86_perf_event_set_period(struct perf_event *event)
> >  		hwc->last_period = period;
> >  		ret = 1;
> >  	}
> > -	/*
> > -	 * Quirk: certain CPUs dont like it if just 1 hw_event is left:
> > -	 */
> > -	if (unlikely(left < 2))
> > -		left = 2;
> >  
> >  	if (left > x86_pmu.max_period)
> >  		left = x86_pmu.max_period;
> >  
> > -	static_call_cond(x86_pmu_limit_period)(event, &left);
> > +	/*
> > +	 * Exempt KVM guest events from the minimum period requirements.  It's
> > +	 * the guest's responsibility to ensure it can make forward progress,
> > +	 * and it's KVM's responsibility to configure an appropriate "period"
> > +	 * to correctly virtualize overflow for the guest's PMCs.
> > +	 */
> > +	if (!event->attr.exclude_host) {
> > +		/*
> > +		 * Quirk: certain CPUs dont like it if just 1 event is left:
> > +		 */
> > +		if (unlikely(left < 2))
> > +			left = 2;
> > +
> > +		static_call_cond(x86_pmu_limit_period)(event, &left);
> > +	}
> >  
> >  	this_cpu_write(pmc_prev_left[idx], left);
> >  
> 
> Nice one. I am curious how you tested this one? I would like to
> reproduce that one on my side.

The check_emulated_instr() sub-test in KVM-Unit-Tests's x86/pmu.c fails when run
with "my" (which is really yours) fix for the KVM's handling of emulated PMC
events[*].  If KVM synthesizes an "instructions retired" event that bumps the
PMC to all ones, i.e. -1 for all intents and purposes, the test fails because
KVM creates a sample_period of '1', but perf programs a period of '2'.

I suspect a very simple test of writing -1 to a PMC from the guest would exhibit
the same behavior.

[*] https://lkml.kernel.org/r/ZUWAg3WP2XESCAR4%40google.com