Re: [PATCH v6 07/16] KVM: x86/pmu: Reprogram PEBS event to emulate guest PEBS counter

[Peter Zijlstra]

On Mon, May 17, 2021 at 07:44:15AM -0700, Andi Kleen wrote:
> 
> On 5/17/2021 1:39 AM, Peter Zijlstra wrote:
> > On Tue, May 11, 2021 at 10:42:05AM +0800, Like Xu wrote:
> > > +	if (pebs) {
> > > +		/*
> > > +		 * The non-zero precision level of guest event makes the ordinary
> > > +		 * guest event becomes a guest PEBS event and triggers the host
> > > +		 * PEBS PMI handler to determine whether the PEBS overflow PMI
> > > +		 * comes from the host counters or the guest.
> > > +		 *
> > > +		 * For most PEBS hardware events, the difference in the software
> > > +		 * precision levels of guest and host PEBS events will not affect
> > > +		 * the accuracy of the PEBS profiling result, because the "event IP"
> > > +		 * in the PEBS record is calibrated on the guest side.
> > > +		 */
> > > +		attr.precise_ip = 1;
> > > +	}
> > You've just destroyed precdist, no?
> 
> precdist can mean multiple things:
> 
> - Convert cycles to the precise INST_RETIRED event. That is not meaningful
> for virtualization because "cycles" doesn't exist, just the raw events.
> 
> - For GLC+ and TNT+ it will force the event to a specific counter that is
> more precise. This would be indeed "destroyed", but right now the patch kit
> only supports Icelake which doesn't support that anyways.
> 
> So I think the code is correct for now, but will need to be changed for
> later CPUs. Should perhaps fix the comment though to discuss this.

OK, can we then do a better comment that explains *why* this is correct
now and what needs help later?

Because IIUC the only reason it is correct now is because:

 - we only support ICL

   * and ICL has pebs_format>=2, so {1,2} are the same
   * and ICL doesn't have precise_ip==3 support

 - Other hardware (GLC+, TNT+) that could possibly care here
   is unsupported atm. but needs changes.

None of which is actually mentioned in that comment it does have.