linux-kernel - Re: [PATCH] perf_event use rdpmc rather than rdmsr when possible in kernel

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Date:	Mon, 27 Feb 2012 12:39:22 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Vince Weaver <vweaver1@...s.utk.edu>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Stephane Eranian <eranian@...il.com>
Subject: Re: [PATCH] perf_event use rdpmc rather than rdmsr when possible in
 kernel

On Mon, 2012-02-20 at 17:38 -0500, Vince Weaver wrote:

> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index 5adce10..5550047 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -85,7 +85,7 @@ u64 x86_perf_event_update(struct perf_event *event)
>  	 */
>  again:
>  	prev_raw_count = local64_read(&hwc->prev_count);
> -	rdmsrl(hwc->event_base, new_raw_count);
> +	new_raw_count=native_read_pmc(hwc->event_base_rdpmc);

That really wants to be rdpmc(), bypassing paravirt like that isn't
nice.

>  
>  	if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
>  					new_raw_count) != prev_raw_count)
> @@ -768,9 +768,11 @@ static inline void x86_assign_hw_event(struct perf_event *event,
>  	} else if (hwc->idx >= X86_PMC_IDX_FIXED) {
>  		hwc->config_base = MSR_ARCH_PERFMON_FIXED_CTR_CTRL;
>  		hwc->event_base = MSR_ARCH_PERFMON_FIXED_CTR0 + (hwc->idx - X86_PMC_IDX_FIXED);
> +		hwc->event_base_rdpmc = (hwc->idx - X86_PMC_IDX_FIXED) | 1<<30;
>  	} else {
>  		hwc->config_base = x86_pmu_config_addr(hwc->idx);
>  		hwc->event_base  = x86_pmu_event_addr(hwc->idx);
> +		hwc->event_base_rdpmc = x86_pmu_addr_offset(hwc->idx);
>  	}
>  }
>  
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index abb2776..432ac69 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -562,6 +562,7 @@ struct hw_perf_event {
>  			u64		last_tag;
>  			unsigned long	config_base;
>  			unsigned long	event_base;
> +			unsigned long	event_base_rdpmc;

We could make that unsigned int, the SDM explicitly states
rdmsr/wrmsr/rdpmc take ECX (and ignore the upper 32bits RCX).

>  			int		idx;
>  			int		last_cpu;
>  			struct hw_perf_event_extra extra_reg;

But yeah, avoiding the extra variable will add a conditional and extra
instructions to the rdpmc path.
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