linux-kernel - Re: [PATCH 2/2] perf/x86/intel: Add Tremont core PMU support

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Date:   Wed, 10 Apr 2019 09:51:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org, tglx@...utronix.de,
        acme@...nel.org, jolsa@...nel.org, eranian@...gle.com,
        alexander.shishkin@...ux.intel.com, ak@...ux.intel.com
Subject: Re: [PATCH 2/2] perf/x86/intel: Add Tremont core PMU support

On Tue, Apr 09, 2019 at 06:10:00PM -0700, kan.liang@...ux.intel.com wrote:

> The generic purpose counter 0 and fixed counter 0 have less skid.
> Force :ppp events on generic purpose counter 0.
> Force instruction:ppp always on fixed counter 0.

> +static struct event_constraint *
> +tnt_get_event_constraints(struct cpu_hw_events *cpuc, int idx,
> +			  struct perf_event *event)
> +{
> +	struct event_constraint *c;
> +
> +	/*
> +	 * :ppp means to do reduced skid PEBS,
> +	 * which is available at PMC0 and fixed counter 0.
> +	 */
> +	if (event->attr.precise_ip == 3) {
> +		/* Force instruction:ppp in Fixed counter 0 */
> +		if (event->hw.config == X86_CONFIG(.event=0xc0))
> +			return &fixed_counter0_constraint;
> +
> +		return &counter0_constraint;

I'm confused, 0xc0 is the architectural 'instructions' event, surely we
can program that on pmc0 too?

Did we want a fixed0_counter0_constraint for that?

> +	}
> +
> +	c = intel_get_event_constraints(cpuc, idx, event);
> +
> +	return c;
> +}