linux-kernel - Re: [PATCH v2] perf/x86/intel: Add support for MISPREDICT bit on Knights Landing cpus

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Date:   Tue, 31 Jul 2018 00:29:25 +0800
From:   Jacek Tomaka <jacekt@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jacek Tomaka <jacek.tomaka@...zta.fm>
Subject: Re: [PATCH v2] perf/x86/intel: Add support for MISPREDICT bit on
 Knights Landing cpus

I do not understand the difference between linear address vs effective
address but LBR_FORMAT_EIP_FLAGS implies effective address, no?

On Tue, Jul 31, 2018 at 12:17 AM, Peter Zijlstra <peterz@...radead.org>
wrote:

> On Mon, Jul 30, 2018 at 10:28:13PM +0800, Jacek Tomaka wrote:
> > From: Jacek Tomaka <jacek.tomaka@...zta.fm>
> >
> > Problem: perf did not show branch predicted/mispredicted bit in brstack.
> >
> > Output of perf -F brstack for profile collected
> >
> > Before:
> > 0x4fdbcd/0x4fdc03/-/-/-/0
> > 0x45f4c1/0x4fdba0/-/-/-/0
> > 0x45f544/0x45f4bb/-/-/-/0
> > 0x45f555/0x45f53c/-/-/-/0
> > 0x7f66901cc24b/0x45f555/-/-/-/0
> > 0x7f66901cc22e/0x7f66901cc23d/-/-/-/0
> > 0x7f66901cc1ff/0x7f66901cc20f/-/-/-/0
> > 0x7f66901cc1e8/0x7f66901cc1fc/-/-/-/0
> >
> > After:
> > 0x4fdbcd/0x4fdc03/P/-/-/0
> > 0x45f4c1/0x4fdba0/P/-/-/0
> > 0x45f544/0x45f4bb/P/-/-/0
> > 0x45f555/0x45f53c/P/-/-/0
> > 0x7f66901cc24b/0x45f555/P/-/-/0
> > 0x7f66901cc22e/0x7f66901cc23d/P/-/-/0
> > 0x7f66901cc1ff/0x7f66901cc20f/P/-/-/0
> > 0x7f66901cc1e8/0x7f66901cc1fc/P/-/-/0
> >
> > Cause:
> > As mentioned in Software Development Manual vol 3, 17.4.8.1,
> > IA32_PERF_CAPABILITIES[5:0] indicates the format of the address that is
> > stored in the LBR stack. Knights Landing reports 1 (LBR_FORMAT_LIP) as
> > its format. Despite that, registers containing FROM address of the
> branch,
> > do have MISPREDICT bit but because of the format indicated in
> > IA32_PERF_CAPABILITIES[5:0], LBR did not read MISPREDICT bit.
> >
> > Solution:
> > Teach LBR about above Knights Landing quirk and make it read MISPREDICT
> bit.
> > ---
> >  arch/x86/events/intel/lbr.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
> > index cf372b9055..043aa09f3a 100644
> > --- a/arch/x86/events/intel/lbr.c
> > +++ b/arch/x86/events/intel/lbr.c
> > @@ -19,7 +19,7 @@ enum {
> >       LBR_FORMAT_MAX_KNOWN    = LBR_FORMAT_TIME,
> >  };
> >
> > -static const enum {
> > +static enum {
> >       LBR_EIP_FLAGS           = 1,
> >       LBR_TSX                 = 2,
> >  } lbr_desc[LBR_FORMAT_MAX_KNOWN + 1] = {
> > @@ -1230,4 +1230,8 @@ void intel_pmu_lbr_init_knl(void)
> >
> >       x86_pmu.lbr_sel_mask = LBR_SEL_MASK;
> >       x86_pmu.lbr_sel_map  = snb_lbr_sel_map;
> > +
> > +     /* Knights Landing does have MISPREDICT bit */
> > +     if (x86_pmu.intel_cap.lbr_format == LBR_FORMAT_LIP)
> > +             lbr_desc[LBR_FORMAT_LIP] |= LBR_EIP_FLAGS;
> >  }
>
> So why not set lbr_format to LBR_FORMAT_EIP_FLAGS ?
>



-- 
*Jacek Tomaka*
Geophysical Software Developer






*DownUnder GeoSolutions*
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West Perth 6005 WA, Australia
*tel *+61 8 9287 4143 <+61%208%209287%204143>
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