Re: [PATCH 3/3] perf, x86, lbr: Demand proper privileges for PERF_SAMPLE_BRANCH_KERNEL

[Stephane Eranian <eranian@google.com>]

On Sat, May 18, 2013 at 12:14 AM, Michael Neuling <mikey@neuling.org> wrote:
> Stephane Eranian <eranian@google.com> wrote:
>
>> On Fri, May 17, 2013 at 1:39 PM, Peter Zijlstra <peterz@infradead.org> wrote:
>> > On Fri, May 17, 2013 at 09:32:08PM +1000, Michael Neuling wrote:
>> >> Peter Zijlstra <peterz@infradead.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.
>
> OK.  I don't have a good handle on what's useful for compilers or JITs
> right now.  I'm just plumbing through what's possible.
>
I understand. It is okay to extend the interface.