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Date: Fri, 06 Aug 2021 20:38:55 +1000
From: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v1 16/55] powerpc/64s: Implement PMU override command line
 option
To: kvm-ppc@vger.kernel.org, Madhavan Srinivasan <maddy@linux.ibm.com>
References: <20210726035036.739609-1-npiggin@gmail.com>
 <20210726035036.739609-17-npiggin@gmail.com>
 <e7bb1311-3b50-dcc2-7fb0-1773558e9abc@linux.ibm.com>
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Excerpts from Madhavan Srinivasan's message of August 6, 2021 5:33 pm:
>=20
> 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/Documenta=
tion/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=3D0x508
>>
>> +	pmu=3D		[PPC] Manually enable the PMU.
>=20
>=20
> This is bit confusing, IIUC, we are manually disabling the perf=20
> 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=20
option. free_run_pmu=3D or something?

Thanks,
Nick

>=20
>=20
>> +			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=3Don'), 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-bo=
ok3s.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 =3D 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=3D 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 =3D true;
>> +
>> +	if (kstrtoul(str, 0, &val))
>> +		val =3D 0;
>> +
>> +	pmu_override_val =3D val;
>> +
>> +	return 1;
>> +}
>> +__setup("pmu=3D", pmu_setup);
>> +
>>   #endif
>=20