linux-kernel - Re: [PATCH 06/12] perf/x86-ibs: Precise event sampling with IBS for AMD CPUs

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Date:	Sat, 14 Apr 2012 12:21:46 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Robert Richter <robert.richter@....com>
Cc:	Ingo Molnar <mingo@...e.hu>, Stephane Eranian <eranian@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/12] perf/x86-ibs: Precise event sampling with IBS for
 AMD CPUs

On Mon, 2012-04-02 at 20:19 +0200, Robert Richter wrote:
> + * We map IBS sampling to following precise levels:
> + *
> + *  1: RIP taken from IBS sample or (if invalid) from stack
> + *  2: RIP always taken from IBS sample, samples with an invalid rip
> + *     are dropped. Thus samples of an event containing two precise
> + *     modifiers (e.g. r076:pp) only contain (precise) addresses
> + *     detected with IBS.

		/*
		 * precise_ip:
		 *
		 *  0 - SAMPLE_IP can have arbitrary skid
		 *  1 - SAMPLE_IP must have constant skid
		 *  2 - SAMPLE_IP requested to have 0 skid
		 *  3 - SAMPLE_IP must have 0 skid
		 *
		 *  See also PERF_RECORD_MISC_EXACT_IP
		 */

your 1 doesn't have constant skid. I would suggest only supporting 2 and
letting userspace drop !PERF_RECORD_MISC_EXACT_IP records if so desired.

That said, mixing the IBS pmu into the regular core pmu isn't exactly
pretty..
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