linux-kernel - Re: [PATCH V7 13/17] perf, x86: enable LBR callstack when recording callchain

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Message-ID: <20141105162932.GX3337@twins.programming.kicks-ass.net>
Date:	Wed, 5 Nov 2014 17:29:32 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Liang, Kan" <kan.liang@...el.com>
Cc:	Stephane Eranian <eranian@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Jiri Olsa <jolsa@...hat.com>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [PATCH V7 13/17] perf, x86: enable LBR callstack when recording
 callchain

On Wed, Nov 05, 2014 at 03:53:34PM +0000, Liang, Kan wrote:
> > I don't think it would be very hard to modify the patch set to make that 3rd
> > mode visible. Just need to make that new PERF_RECORD_* type visible to
> > user and modify the compatibility checks.
> 
> It's not hard. But LBR is not an independent callchain options. It's better to be
> a supplement of FP. Otherwise, it may confuse the user. He enables the
> BRANCH_CALL_STACK, but the data is partly or even not at all from hardware.

What the user sees is up to userspace. It should not be forced by the
kernel/user interface.
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