linux-kernel - Re: [PATCH v9 1/3] arm64: perf: Add userspace counter access disable switch

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Date:   Tue, 24 Aug 2021 16:26:22 +0100
From:   Will Deacon <will@...nel.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        honnappa.nagarahalli@....com, Zachary.Leaf@....com,
        Raphael Gault <raphael.gault@....com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Itaru Kitayama <itaru.kitayama@...il.com>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v9 1/3] arm64: perf: Add userspace counter access disable
 switch

On Fri, Aug 06, 2021 at 04:51:21PM -0600, Rob Herring wrote:
> Like x86, some users may want to disable userspace PMU counter
> altogether. Add a sysctl 'perf_user_access' file to control userspace
> counter access. The default is '0' which is disabled. Writing '1'
> enables access.
> 
> Note that x86 also supports writing '2' to globally enable user access.
> As there's not existing userspace support to worry about, this shouldn't
> be necessary for Arm. It could be added later if the need arises.
> 
> Cc: Will Deacon <will@...nel.org>
> Cc: Mark Rutland <mark.rutland@....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>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: linux-perf-users@...r.kernel.org
> Signed-off-by: Rob Herring <robh@...nel.org>
> ---
> v9:
>  - Use sysctl instead of sysfs attr
>  - Default to disabled
> v8:
>  - New patch
> 
> ---
>  arch/arm64/kernel/perf_event.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
> index d07788dad388..74f77b68f5f0 100644
> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -286,6 +286,21 @@ static const struct attribute_group armv8_pmuv3_events_attr_group = {
>  PMU_FORMAT_ATTR(event, "config:0-15");
>  PMU_FORMAT_ATTR(long, "config1:0");
>  
> +static int sysctl_perf_user_access __read_mostly;
> +
> +static struct ctl_table armv8_pmu_sysctl_table[] = {
> +	{
> +		.procname       = "perf_user_access",
> +		.data		= &sysctl_perf_user_access,
> +		.maxlen		= sizeof(unsigned int),
> +		.mode           = 0644,
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= SYSCTL_ZERO,
> +		.extra2		= SYSCTL_ONE,
> +	},
> +	{ }
> +};

This should be documented somewhere. Maybe add an entry to
Documentation/admin-guide/sysctl/kernel.rst which points at the doc in patch
3, which needs updating to talk about this control?

Otherwise, looks good:

Acked-by: Will Deacon <will@...nel.org>

Will