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

[Peter Zijlstra]

On Mon, Jul 30, 2018 at 10:28:13PM +0800, Jacek Tomaka wrote:
> From: Jacek Tomaka <jacek.tomaka@xxxxxxxxx>
> 
> 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 ?