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

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Date:	Tue, 21 May 2013 15:41:35 +1000
From:	Michael Neuling <mikey@...ling.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	anshuman Stephane Eranian <eranian@...gle.com>,
	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

Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, May 16, 2013 at 05:36:11PM +0200, Stephane Eranian wrote:
> > On Thu, May 16, 2013 at 1:16 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > > On Thu, May 16, 2013 at 08:15:17PM +1000, Michael Neuling wrote:
> > >> Peter,
> > >>
> > >> BTW PowerPC also has the ability to filter on conditional branches.  Any
> > >> chance we could add something like the follow to perf also?
> > >>
> > >
> > > I don't see an immediate problem with that except that we on x86 need to
> > > implement that in the software filter. Stephane do you see any
> > > fundamental issue with that?
> > >
> > On X86, the LBR cannot filter on conditional in HW. Thus as Peter said, it would
> > have to be done in SW. I did not add that because I think those branches are
> > not necessarily useful for tools.
> 
> 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?
> 
> 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.
> 
> Also, it looks like its trivial to add to x86, you seem to have already done
> all the hard work by having X86_BR_JCC.
> 
> The only missing piece would be:

Peter,

Can we add your signed-off-by on this?

We are cleaning up our series for conditional branches and would like to
add this as part of the post.

Mikey


> 
> --- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> @@ -337,6 +337,10 @@ static int intel_pmu_setup_sw_lbr_filter
>  
>  	if (br_type & PERF_SAMPLE_BRANCH_IND_CALL)
>  		mask |= X86_BR_IND_CALL;
> +
> +	if (br_type & PERF_SAMPLE_BRANCH_CONDITIONAL)
> +		mask |= X86_BR_JCC;
> +
>  	/*
>  	 * stash actual user request into reg, it may
>  	 * be used by fixup code for some CPU
> 
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