linux-kernel - Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics

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Date:   Sat, 31 Aug 2019 11:19:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     kan.liang@...ux.intel.com, acme@...nel.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, jolsa@...nel.org,
        eranian@...gle.com, alexander.shishkin@...ux.intel.com
Subject: Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown
 metrics

On Wed, Aug 28, 2019 at 12:04:45PM -0700, Andi Kleen wrote:

> > > NMI
> > > ======
> > > 
> > > The METRICS register may be overflow. The bit 48 of STATUS register
> > > will be set. If so, update all active slots and metrics events.
> > 
> > that happen? It would be useful to get that METRIC_OVF (can we please
> 
> This happens when the internal counters that feed the metrics
> overflow.
> 
> > If this is so; then we can use this to update/reset PERF_METRICS and
> > nothing else.
> 
> It has to be handled in the PMI.

That's what I wrote; Overflow is always NMI.

> > Then there is no mucking about with that odd counter/metrics msr pair
> > reset nonsense. Becuase that really stinks.
> 
> You have to write them to reset the internal counters.

But not for ever read, only on METRIC_OVF.