[PATCH v1 16/55] powerpc/64s: Implement PMU override command line option

[Nicholas Piggin]

Excerpts from Athira Rajeev's message of August 6, 2021 7:28 pm:
> 
> 
>> On 26-Jul-2021, at 9:19 AM, Nicholas Piggin <npiggin at gmail.com> wrote:
>> 
>> It can be useful in simulators (with very constrained environments)
>> to allow some PMCs to run from boot so they can be sampled directly
>> by a test harness, rather than having to run perf.
>> 
>> A previous change freezes counters at boot by default, so provide
>> a boot time option to un-freeze (plus a bit more flexibility).
>> 
>> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
>> ---
>> .../admin-guide/kernel-parameters.txt         |  7 ++++
>> arch/powerpc/perf/core-book3s.c               | 35 +++++++++++++++++++
>> 2 files changed, 42 insertions(+)
>> 
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index bdb22006f713..96b7d0ebaa40 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -4089,6 +4089,13 @@
>> 			Override pmtimer IOPort with a hex value.
>> 			e.g. pmtmr=0x508
>> 
>> +	pmu=		[PPC] Manually enable the PMU.
>> +			Enable the PMU by setting MMCR0 to 0 (clear FC bit).
>> +			This option is implemented for Book3S processors.
>> +			If a number is given, then MMCR1 is set to that number,
>> +			otherwise (e.g., 'pmu=on'), it is left 0. The perf
>> +			subsystem is disabled if this option is used.
>> +
>> 	pm_debug_messages	[SUSPEND,KNL]
>> 			Enable suspend/resume debug messages during boot up.
>> 
>> diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
>> index 65795cadb475..e7cef4fe17d7 100644
>> --- a/arch/powerpc/perf/core-book3s.c
>> +++ b/arch/powerpc/perf/core-book3s.c
>> @@ -2428,8 +2428,24 @@ int register_power_pmu(struct power_pmu *pmu)
>> }
>> 
>> #ifdef CONFIG_PPC64
>> +static bool pmu_override = false;
>> +static unsigned long pmu_override_val;
>> +static void do_pmu_override(void *data)
>> +{
>> +	ppc_set_pmu_inuse(1);
>> +	if (pmu_override_val)
>> +		mtspr(SPRN_MMCR1, pmu_override_val);
>> +	mtspr(SPRN_MMCR0, mfspr(SPRN_MMCR0) & ~MMCR0_FC);
> 
> Hi Nick
> 
> Here, we are not doing any validity check for the value used to set MMCR1. 
> For advanced users, the option to pass value for MMCR1 is fine. But other cases, it could result in
> invalid event getting used. Do we need to restrict this boot time option for only PMC5/6 ?

Depends what would be useful. We don't have to prevent the admin shooting 
themselves in the foot with options like this, but if we can make it 
safer without making it less useful then that's always a good option.

Thanks,
Nick