linux-kernel - Re: [PATCH V6] arm64: perf: Make exporting of pmu events configurable

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Date:   Fri, 24 Jun 2022 13:18:26 +0100
From:   Will Deacon <will@...nel.org>
To:     Srinivasarao Pathipati <quic_c_spathi@...cinc.com>
Cc:     mark.rutland@....com, peterz@...radead.org, mingo@...hat.com,
        acme@...nel.org, alexander.shishkin@...ux.intel.com,
        jolsa@...nel.org, namhyung@...nel.org, catalin.marinas@....com,
        linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V6] arm64: perf: Make exporting of pmu events configurable

On Thu, Jun 09, 2022 at 06:29:49PM +0530, Srinivasarao Pathipati wrote:
> The PMU export bit (PMCR_EL0.X) is getting reset during pmu reset,
> Make it configurable using sysctls to enable/disable at runtime.
> 
> Signed-off-by: Srinivasarao Pathipati <quic_c_spathi@...cinc.com>
> ---
> Changes since V5:
> 	- removed configuring with kernel parameters.
> Changes since V4:
> 	- Registering sysctls dynamically for only arm64 as suggested by Will
> 	- Not removed the code to configure with kernel parameters 
> 	  as the sysctl's kernel parameter(sysctl.kernel.export_pmu_events)
> 	  is not working at early bootup. pmu_reset() getting called before 
> 	  sysctl's kernel parameter is set.
> Changes since V3:
> 	- export bit is now configurable with sysctl
> 	- enabling export bit on reset instead of retaining
> 
> Changes since V2:
> 	Done below changes as per Will's comments
> 	- enabling pmcr_x now configurable with kernel parameters and
> 	  by default it is disabled.
> 	
> Changes since V1:
> 	- Preserving only PMCR_X bit as per Robin Murphy's comment.
> 
> ---
>  Documentation/admin-guide/sysctl/kernel.rst | 11 +++++++++++
>  arch/arm64/kernel/perf_event.c              | 13 +++++++++++++
>  2 files changed, 24 insertions(+)
> 
> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index ddccd10..c2ecd84 100644
> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -267,6 +267,17 @@ domain names are in general different. For a detailed discussion
>  see the ``hostname(1)`` man page.
>  
>  
> +export_pmu_events (arm64 only)
> +==============================
> +
> +Controls the PMU export bit (PMCR_EL0.X), which enables the exporting of
> +events over an IMPLEMENTATION DEFINED PMU event export bus to another device.
> +
> +0: disables exporting of events (default).
> +
> +1: enables exporting of events.
> +
> +
>  firmware_config
>  ===============
>  
> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
> index cb69ff1..a8c32a0 100644
> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -298,6 +298,7 @@ PMU_FORMAT_ATTR(long, "config1:0");
>  PMU_FORMAT_ATTR(rdpmc, "config1:1");
>  
>  static int sysctl_perf_user_access __read_mostly;
> +static int sysctl_export_pmu_events __read_mostly;
>  
>  static inline bool armv8pmu_event_is_64bit(struct perf_event *event)
>  {
> @@ -1047,6 +1048,9 @@ static void armv8pmu_reset(void *info)
>  	if (armv8pmu_has_long_event(cpu_pmu))
>  		pmcr |= ARMV8_PMU_PMCR_LP;
>  
> +	if (sysctl_export_pmu_events)
> +		pmcr |= ARMV8_PMU_PMCR_X;

I think we need to do this in armv8pmu_start() rather than armv8pmu_reset(),
otherwise any changes to the sysctl at runtime won't take effect unless you
do something like re-online the CPU.

Will