Re: [PATCH] drivers/perf: arm_spe: Fix consistency of SYS_PMSCR_EL1.CX

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 12/07/2022 06:14, Anshuman Khandual wrote:
> The arm_spe_pmu driver will enable SYS_PMSCR_EL1.CX in order to add CONTEXT
> packets into the traces, if the owner of the perf event runs with required
> capabilities i.e CAP_PERFMON or CAP_SYS_ADMIN via perfmon_capable() helper.
> 
> The value of this bit is computed in the arm_spe_event_to_pmscr() function
> but the check for capabilities happens in the pmu event init callback i.e
> arm_spe_pmu_event_init(). This suggests that the value of the CX bit should
> remain consistent for the duration of the perf session.
> 
> However, the function arm_spe_event_to_pmscr() may be called later during
> the event start callback i.e arm_spe_pmu_start() when the "current" process
> is not the owner of the perf session, hence the CX bit setting is currently
> not consistent.
> 
> One way to fix this, is by caching the required value of the CX bit during
> the initialization of the PMU event, so that it remains consistent for the
> duration of the session. It uses currently unused 'event->hw.flags' element
> to cache perfmon_capable() value, which can be referred during event start
> callback to compute SYS_PMSCR_EL1.CX. This ensures consistent availability
> of context packets in the trace as per event owner capabilities.
> 
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> This applies on v5.19-rc6 and built on an earlier version posted by German
> https://lore.kernel.org/all/20220117124432.3119132-1-german.gomez@arm.com/
> 
>   drivers/perf/arm_spe_pmu.c | 25 +++++++++++++++++++++++--
>   1 file changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
> index db670b265897..011e98428233 100644
> --- a/drivers/perf/arm_spe_pmu.c
> +++ b/drivers/perf/arm_spe_pmu.c
> @@ -39,6 +39,26 @@
>   #include <asm/mmu.h>
>   #include <asm/sysreg.h>
>   
> +/*
> + * event.hw.flags remain unused for events created for this
> + * PMU driver. A single bit there i.e BIT(0), could be used
> + * to remember initiating process's perfmon_capable() value
> + * which can be subsequently used to enable SYS_PMSCR_EL.CX
> + * thus enabling context information in the trace.

Please could we rephrase this :

  /*
   * Cache if the event is allowed to trace Context information.
   * This allows us to perform the check, i.e, perfmon_capable(),
   * in the context of the event owner, once, during the event_init().
   */

> + */
> +#define SPE_PMU_HW_FLAGS_CX			BIT(0)
> +
> +static void event_hw_flags_set_cx(struct perf_event *event)
> +{
> +	if (perfmon_capable())
> +		event->hw.flags |= SPE_PMU_HW_FLAGS_CX;
> +}
> +
> +static bool event_hw_flags_has_cx(struct perf_event *event)
> +{
> +	return !!(event->hw.flags & SPE_PMU_HW_FLAGS_CX);
> +}
> +

super minor nit:

	set_event_has_cx();
	get_event_has_cx();
?

Also, please could we fold the CONFIG_PID_IN_CONTEXTIDR check
into the helpers ?

>   #define ARM_SPE_BUF_PAD_BYTE			0
>   
>   struct arm_spe_pmu_buf {
> @@ -272,7 +292,7 @@ static u64 arm_spe_event_to_pmscr(struct perf_event *event)
>   	if (!attr->exclude_kernel)
>   		reg |= BIT(SYS_PMSCR_EL1_E1SPE_SHIFT);
>   
> -	if (IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR) && perfmon_capable())
> +	if (IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR) && event_hw_flags_has_cx(event))
>   		reg |= BIT(SYS_PMSCR_EL1_CX_SHIFT);

i.e,
	if (event_has_cx(event))
		....

Otherwise looks good to me.

Suzuki