linux-kernel - Re: [PATCH 1/6] perf, x86: Basic Haswell LBR call stack support

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Message-ID: <50862B95.4020105@intel.com>
Date:	Tue, 23 Oct 2012 13:31:01 +0800
From:	"Yan, Zheng" <zheng.z.yan@...el.com>
To:	Stephane Eranian <eranian@...gle.com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [PATCH 1/6] perf, x86: Basic Haswell LBR call stack support

On 10/22/2012 07:23 PM, Stephane Eranian wrote:
> On Mon, Oct 22, 2012 at 12:33 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>>
>> On Mon, 2012-10-22 at 14:11 +0800, Yan, Zheng wrote:
>>> +       /* LBR callstack does not work well with FREEZE_LBRS_ON_PMI */
>>> +       if (!cpuc->lbr_sel || !(cpuc->lbr_sel->config & LBR_CALL_STACK))
>>> +               debugctl |= DEBUGCTLMSR_FREEZE_LBRS_ON_PMI;
>>
> Is this a bug or a feature?
> 
It's a hardware bug. If FREEZE_LBRS_ON_PMI is set, PMI may corrupt the call stack records.

Regards
Yan, Zheng

>>
>> How useful it is without this? How many calls between PMI and us getting
>> to intel_pmu_lbr_read()?
> 
> Agreed with Peter. Freeze on PMI is critical for LBR in general.
> 

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