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

[Andi Kleen]

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.


-Andi