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

[Xu, Like]

On 2021/5/18 16:47, Peter Zijlstra wrote:
> 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.

Hi Andi & Peter,

By "precdist", do you mean the"Precise Distribution of Instructions Retired 
(PDIR) Facility"?

The SDM says Ice Lake Microarchitecture does support PEBS-PDIR on 
IA32_FIXED0 only.
And this patch kit enables it in the patch 0011, please take a look.

Or do I miss something about precdist on ICL ?

Thanks,
Like Xu