linux-kernel - Re: [PATCH] arm_pmu: Fix write counter incorrect in ARMv7 big-endian mode

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Message-ID: <20210430123408.GD53304@C02TD0UTHF1T.local>
Date:   Fri, 30 Apr 2021 13:34:08 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Yang Jihong <yangjihong1@...wei.com>, will@...nel.org
Cc:     peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, linux@...linux.org.uk, suzuki.poulose@....com,
        julien.thierry.kdev@...il.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm_pmu: Fix write counter incorrect in ARMv7 big-endian
 mode

On Fri, Apr 30, 2021 at 09:26:59AM +0800, Yang Jihong wrote:
> Commit 3a95200d3f89a ("arm_pmu: Change API to support 64bit counter values")
> changes the input "value" type from 32-bit to 64-bit,
> which introduces the following problem:
> ARMv7 PMU counters is 32-bit width, in big-endian mode, write counter uses
> high 32-bit, which writes an incorrect value.

It might be worth noting that the reason for this is that when a 64-bit
value is split across two 32-bit registers, the high/low halves are
split to match how LDRD would load a 64-bit quantity (and so differs
across big/little endian), but the "r" constraint consistently uses the
first of the two regiseters.

Given that, this patch makes sense to me (and I didn't spot any related
issues), so:

Acked-by: Mark Rutland <mark.rutland@....com>

Will, I assume you'll pick this up.

Mark.

> 
> Before:
> 
>  Performance counter stats for 'ls':
> 
>               2.22 msec task-clock                #    0.675 CPUs utilized
>                  0      context-switches          #    0.000 K/sec
>                  0      cpu-migrations            #    0.000 K/sec
>                 49      page-faults               #    0.022 M/sec
>         2150476593      cycles                    #  966.663 GHz
>         2148588788      instructions              #    1.00  insn per cycle
>         2147745484      branches                  # 965435.074 M/sec
>         2147508540      branch-misses             #   99.99% of all branches
> 
> None of the above hw event counters are correct.
> 
> Solution:
> 
> "value" forcibly converted to 32-bit type before being written to PMU register.
> 
> After:
> 
>  Performance counter stats for 'ls':
> 
>               2.09 msec task-clock                #    0.681 CPUs utilized
>                  0      context-switches          #    0.000 K/sec
>                  0      cpu-migrations            #    0.000 K/sec
>                 46      page-faults               #    0.022 M/sec
>            2807301      cycles                    #    1.344 GHz
>            1060159      instructions              #    0.38  insn per cycle
>             250496      branches                  #  119.914 M/sec
>              23192      branch-misses             #    9.26% of all branches
> 
> Fixes: 3a95200d3f89a ("arm_pmu: Change API to support 64bit counter values")
> Signed-off-by: Yang Jihong <yangjihong1@...wei.com>
> ---
>  arch/arm/kernel/perf_event_v7.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/kernel/perf_event_v7.c b/arch/arm/kernel/perf_event_v7.c
> index 2924d7910b10..eb2190477da1 100644
> --- a/arch/arm/kernel/perf_event_v7.c
> +++ b/arch/arm/kernel/perf_event_v7.c
> @@ -773,10 +773,10 @@ static inline void armv7pmu_write_counter(struct perf_event *event, u64 value)
>  		pr_err("CPU%u writing wrong counter %d\n",
>  			smp_processor_id(), idx);
>  	} else if (idx == ARMV7_IDX_CYCLE_COUNTER) {
> -		asm volatile("mcr p15, 0, %0, c9, c13, 0" : : "r" (value));
> +		asm volatile("mcr p15, 0, %0, c9, c13, 0" : : "r" ((u32)value));
>  	} else {
>  		armv7_pmnc_select_counter(idx);
> -		asm volatile("mcr p15, 0, %0, c9, c13, 2" : : "r" (value));
> +		asm volatile("mcr p15, 0, %0, c9, c13, 2" : : "r" ((u32)value));
>  	}
>  }
>  
> -- 
> 2.30.GIT
>