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

[Will Deacon <will@kernel.org>]

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@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Jiri Olsa <jolsa@redhat.com>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: linux-perf-users@vger.kernel.org
> Signed-off-by: Rob Herring <robh@kernel.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@kernel.org>

Will