linux-kernel - Re: [PATCH] perf: Change PMCR write to read-modify-write

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Date:   Thu, 21 Mar 2019 13:34:55 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Prasad Sodagudi <psodagud@...eaurora.org>, will.deacon@....com,
        mingo@...hat.com, catalin.marinas@....com,
        linux-arm-kernel@...ts.infradead.org, peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: Change PMCR write to read-modify-write

Hi Prasad,

On 21/03/2019 02:07, Prasad Sodagudi wrote:
> Preserves the bitfields of PMCR_EL0(AArch64) during PMU reset.
> Reset routine should write a 1 to PMCR.C and PMCR.P fields only
> to reset the counters. Other fields should not be changed
> as they could be set before PMU initialization and their
> value must be preserved even after reset.
> 

Are there any particular bit you are concerned about? Apart from the RO
ones and the Res0 ones (to which we are already writing 0), I see:

DP -> irrelevant for non-secure
X -> This one is the only potentially interesting, however it resets to
an architecturally unknown value, so unless we know for a fact it was
set before hand, we probably want to clear it
D -> ignored when we have LC set (and we do)
E -> Since this is the function we use to reset the PMU on the current
CPU, we probably want to set this bit to 0 regardless of its previous value

So, is there any issue this patch is solving?

Thanks,

> Signed-off-by: Prasad Sodagudi <psodagud@...eaurora.org>
> ---
>  arch/arm64/kernel/perf_event.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
> index 4addb38..0c1afdd 100644
> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -868,8 +868,8 @@ static void armv8pmu_reset(void *info)
>  	 * Initialize & Reset PMNC. Request overflow interrupt for
>  	 * 64 bit cycle counter but cheat in armv8pmu_write_counter().
>  	 */
> -	armv8pmu_pmcr_write(ARMV8_PMU_PMCR_P | ARMV8_PMU_PMCR_C |
> -			    ARMV8_PMU_PMCR_LC);
> +	armv8pmu_pmcr_write(armv8pmu_pmcr_read() | ARMV8_PMU_PMCR_P |
> +			ARMV8_PMU_PMCR_C | ARMV8_PMU_PMCR_LC);
>  }
>  
>  static int __armv8_pmuv3_map_event(struct perf_event *event,
> 

-- 
Julien Thierry