linux-kernel - Re: [PATCH v2 2/5] perf/x86: add support for sampling PEBS machine state registers

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Date:	Tue, 15 Jul 2014 16:29:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, ak@...ux.intel.com,
	jolsa@...hat.com, acme@...hat.com, namhyung@...nel.org
Subject: Re: [PATCH v2 2/5] perf/x86: add support for sampling PEBS machine
 state registers

On Tue, Jul 15, 2014 at 02:31:41AM +0200, Stephane Eranian wrote:
> PEBS can capture machine state regs at retiremnt of the sampled
> instructions. When precise sampling is enabled on an event, PEBS
> is used, so substitute the interrupted state with the PEBS state.
> Note that not all registers are captured by PEBS. Those missing
> are replaced by the interrupt state counter-parts.
> 
> Signed-off-by: Stephane Eranian <eranian@...gle.com>
> ---
>  arch/x86/kernel/cpu/perf_event_intel_ds.c |   19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event_intel_ds.c b/arch/x86/kernel/cpu/perf_event_intel_ds.c
> index 980970c..d612bcd 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel_ds.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c
> @@ -925,6 +925,25 @@ static void __intel_pmu_pebs_event(struct perf_event *event,
>  	regs.bp = pebs->bp;
>  	regs.sp = pebs->sp;
>  
> +	if (sample_type & PERF_SAMPLE_REGS_INTR) {
> +		regs.ax = pebs->ax;
> +		regs.bx = pebs->bx;
> +		regs.cx = pebs->cx;
> +		regs.si = pebs->si;
> +		regs.di = pebs->di;
> +
> +		regs.r8 = pebs->r8;
> +		regs.r9 = pebs->r9;
> +		regs.r10 = pebs->r10;
> +		regs.r11 = pebs->r11;
> +		regs.r12 = pebs->r12;
> +		regs.r13 = pebs->r13;
> +		regs.r14 = pebs->r14;
> +		regs.r14 = pebs->r15;
> +
> +		data.regs_intr.regs = &regs;

This last assignment is superfluous. Note how you previous patch's
perf_sample_regs_intr() is unconditionally writing its .regs value, and
in turn, perf_sample_regs_intr() is called unconditionally from
perf_prepare_sample().



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