Re: [PATCH 2/9] perf/x86/intel: Basic support for metrics counters

[Peter Zijlstra]

On Tue, May 21, 2019 at 02:40:48PM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> 
> Metrics counters (hardware counters containing multiple metrics)
> are modeled as separate registers for each TopDown metric events,
> with an extra reg being used for coordinating access to the
> underlying register in the scheduler.
> 
> This patch adds the basic infrastructure to separate the scheduler
> register indexes from the actual hardware register indexes. In
> most cases the MSR address is already used correctly, but for
> code using indexes we need a separate reg_idx field in the event
> to indicate the correct underlying register.

That doesn't parse. What exactly is the difference between reg_idx and
idx? AFAICT there is a fixed relation like:

  reg_idx = is_metric_idx(idx) ? INTEL_PMC_IDX_FIXED_SLOTS : idx;

Do we really need a variable for that?

Also, why do we need that whole enabled_events[] array. Do we really not
have that information elsewhere?

I shouldn've have to reverse engineer patches :/