linux-kernel - Re: [PATCH 3/3] perf, x86, lbr: Demand proper privileges for PERF_SAMPLE_BRANCH

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Date:	Fri, 17 May 2013 23:39:46 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Michael Neuling <mikey@...ling.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Michael Ellerman <michael@...erman.id.au>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	Linux PPC dev <linuxppc-dev@...abs.org>
Subject: Re: [PATCH 3/3] perf, x86, lbr: Demand proper privileges for PERF_SAMPLE_BRANCH_KERNEL

On Fri, May 17, 2013 at 1:39 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, May 17, 2013 at 09:32:08PM +1000, Michael Neuling wrote:
>> Peter Zijlstra <peterz@...radead.org> wrote:
>
>> > Wouldn't it be mostly conditional branches that are the primary control flow
>> > and can get predicted wrong? I mean, I'm sure someone will miss-predict an
>> > unconditional branch but its not like we care about people with such
>> > afflictions do we?
>>
>> You could mispredict the target address of a computed goto.  You'd know
>> it was taken but not know target address until later in the pipeline.
>
> Oh right, computed targets could indeed be mis predicted. I was more thinking
> about jumps with immediate values.
>
>> On this, the POWER8 branch history buffer tells us two things about the
>> prediction status.
>>   1) if the branch was predicted taken/not taken correctly
>>   2) if the target address was predicted correctly or not (for computed
>>      gotos only)
>> So we'd actually like more prediction bits too :-D
>
> So if I understand this right, 1) maps to the predicted flags we have; 2)
> would be new stuff?
>
> We don't really have anything like that on x86, but I suppose if you make the
> thing optional and present a 'useful' use-case implemented in userspace code
> we could take it :-)
>
>> > Anyway, since PPC people thought it worth baking into hardware,
>> > presumably they have a compelling use case. Mikey could you see if you
>> > can retrieve that from someone in the know? It might be interesting.
>>
>> I don't think we can mispredict a non-conditional non-computed but I'll
>> have to check with the HW folks.
>
> I was mostly wondering about the use-case for the conditional filter. Stephane
> didn't think it useful, clearly your hardware guys thought different :-)

>From my experience talking with compiler people, they care about ALL
the branches
and not the conditional so much. They use LBR to do basic block profiling.
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