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

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Madhavan Srinivasan's message of August 6, 2021 5:33 pm:
> 
> On 7/26/21 9:19 AM, Nicholas Piggin 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@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.
> 
> 
> This is bit confusing, IIUC, we are manually disabling the perf 
> registration
> with this option and not pmu.
> If this option is used, we will unfreeze the
> MMCR0_FC (only in the HV_mode) and not register perf subsystem.

With the previous patch, this option un-freezes the PMU
(and disables perf).

> Since this option is valid only for HV_mode, canwe call it
> kvm_disable_perf or kvm_dis_perf.

It's only disabled for guests because it would require a bit
of logic to set pmcregs_in_use when we register our lppaca. We could
add that if needed, but the intention is for use on BML, not exactly
KVM specific.

I can add HV restriction to the help text. And we could rename the 
option. free_run_pmu= or something?

Thanks,
Nick

> 
> 
>> +			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);
>> +}
>> +
>>   static int __init init_ppc64_pmu(void)
>>   {
>> +	if (cpu_has_feature(CPU_FTR_HVMODE) && pmu_override) {
>> +		printk(KERN_WARNING "perf: disabling perf due to pmu= command line option.\n");
>> +		on_each_cpu(do_pmu_override, NULL, 1);
>> +		return 0;
>> +	}
>> +
>>   	/* run through all the pmu drivers one at a time */
>>   	if (!init_power5_pmu())
>>   		return 0;
>> @@ -2451,4 +2467,23 @@ static int __init init_ppc64_pmu(void)
>>   		return init_generic_compat_pmu();
>>   }
>>   early_initcall(init_ppc64_pmu);
>> +
>> +static int __init pmu_setup(char *str)
>> +{
>> +	unsigned long val;
>> +
>> +	if (!early_cpu_has_feature(CPU_FTR_HVMODE))
>> +		return 0;
>> +
>> +	pmu_override = true;
>> +
>> +	if (kstrtoul(str, 0, &val))
>> +		val = 0;
>> +
>> +	pmu_override_val = val;
>> +
>> +	return 1;
>> +}
>> +__setup("pmu=", pmu_setup);
>> +
>>   #endif
>