linux-kernel - RE: [PATCH V3 1/3] perf tools: enable LBR call stack support

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Message-ID: <37D7C6CF3E00A74B8858931C1DB2F077016711E8@SHSMSX103.ccr.corp.intel.com>
Date:	Tue, 18 Nov 2014 13:57:12 +0000
From:	"Liang, Kan" <kan.liang@...el.com>
To:	Namhyung Kim <namhyung@...nel.org>
CC:	"acme@...nel.org" <acme@...nel.org>,
	"jolsa@...hat.com" <jolsa@...hat.com>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"eranian@...gle.com" <eranian@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: RE: [PATCH V3 1/3] perf tools: enable LBR call stack support


> 
> On Fri, 14 Nov 2014 08:44:10 -0500, kan liang wrote:
> > From: Kan Liang <kan.liang@...el.com>
> >
> > Currently, there are two call chain recording options, fp and dwarf.
> > Haswell has a new feature that utilizes the existing LBR facility to
> > record call chains. So it provides the third options to record call
> > chain. This patch enables the lbr call stack support.
> >
> > LBR call stack has some limitations. It reuses current LBR facility,
> > so LBR call stack and branch record can not be enabled at the same
> > time. It is only available for user callchain.
> > However, LBR call stack can work on the user app which doesn't have
> > frame-pointer or dwarf debug info compiled. It is a good alternative
> > when nothing else works.
> 
> So this is only for Haswell or later, right?  

Right.

>What if user gives it to the other
> CPUs - is it fail or ignore silently?  I think we need to warn user about it
> more kindly..

The kernel will handle it. EOPNOTSUPP will be return if there is no LBR
Call stack support.

Thanks,
Kan

> 
> Thanks,
> Namhyung
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