linux-kernel - Re: [PATCH] arm64: perf: Fix 64-bit event counter read truncation

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Date:   Wed, 10 Mar 2021 10:53:36 +0000
From:   Alexandru Elisei <alexandru.elisei@....com>
To:     Rob Herring <robh@...nel.org>, Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH] arm64: perf: Fix 64-bit event counter read truncation

Hi Rob,

On 3/10/21 12:44 AM, Rob Herring wrote:
> Commit 0fdf1bb75953 ("arm64: perf: Avoid PMXEV* indirection") changed
> armv8pmu_read_evcntr() to return a u32 instead of u64. The result is
> silent truncation of the event counter when using 64-bit counters. Given
> the offending commit appears to have passed thru several folks, it seems
> likely this was a bad rebase after v8.5 PMU 64-bit counters landed.

Thank you for the fix, it does seem that I made a mistake when rebasing the
series. Version v4 of the PMU NMI series was sent in 2019, then patch 8673e02e5841
("arm64: perf: Add support for ARMv8.5-PMU 64-bit counters") from March 2020
changed the read of PMEVCNTR_EL0 to return an u64, then version v5 from June 2020
changed it back to returning an u32.

The result of read_pmvevcntr() is returned by armv8pmu_read_evcntr(), and it is an
unsigned long which is 64bits for arm64, so the patch looks good to me:

Reviewed-by: Alexandru Elisei <alexandru.elisei@....com>

Thanks,

Alex

>
> Fixes: 0fdf1bb75953 ("arm64: perf: Avoid PMXEV* indirection")
> Cc: Alexandru Elisei <alexandru.elisei@....com>
> Cc: Julien Thierry <julien.thierry.kdev@...il.com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
> Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> Cc: Jiri Olsa <jolsa@...hat.com>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Signed-off-by: Rob Herring <robh@...nel.org>
> ---
>  arch/arm64/kernel/perf_event.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
> index 7d2318f80955..4658fcf88c2b 100644
> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -460,7 +460,7 @@ static inline int armv8pmu_counter_has_overflowed(u32 pmnc, int idx)
>  	return pmnc & BIT(ARMV8_IDX_TO_COUNTER(idx));
>  }
>  
> -static inline u32 armv8pmu_read_evcntr(int idx)
> +static inline u64 armv8pmu_read_evcntr(int idx)
>  {
>  	u32 counter = ARMV8_IDX_TO_COUNTER(idx);
>