Re: [RFC V1 11/11] perf: Capture branch privilege information

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 1/26/22 22:57, James Clark wrote:
> 
> On 24/01/2022 04:30, Anshuman Khandual wrote:
>> Platforms like arm64 could capture privilege level information for all the
>> branch records. Hence this adds a new element in the struct branch_entry to
>> record the privilege level information, which could be requested through a
>> new event.attr.branch_sample_type flag PERF_SAMPLE_BRANCH_PRIV_SAVE. While
>> here, update the BRBE driver as required.
>>
>> 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: Jiri Olsa <jolsa@redhat.com>
>> Cc: Namhyung Kim <namhyung@kernel.org>
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-perf-users@vger.kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>>  drivers/perf/arm_pmu_brbe.c              | 28 ++++++++++++++++++++++++
>>  include/linux/perf_event.h               |  5 +++++
>>  include/uapi/linux/perf_event.h          | 13 ++++++++++-
>>  tools/include/uapi/linux/perf_event.h    | 13 ++++++++++-
>>  tools/perf/Documentation/perf-record.txt |  1 +
>>  tools/perf/util/parse-branch-options.c   |  1 +
>>  6 files changed, 59 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/perf/arm_pmu_brbe.c b/drivers/perf/arm_pmu_brbe.c
>> index 7cd1208c6c58..d4cbea74c148 100644
>> --- a/drivers/perf/arm_pmu_brbe.c
>> +++ b/drivers/perf/arm_pmu_brbe.c
>> @@ -270,6 +270,25 @@ static int brbe_fetch_perf_type(u64 brbinf)
>>  	}
>>  }
>>  
>> +static int brbe_fetch_perf_priv(u64 brbinf)
>> +{
>> +	int brbe_el = brbe_fetch_el(brbinf);
>> +
>> +	switch (brbe_el) {
>> +	case BRBINF_EL_EL0:
>> +		return PERF_BR_USER;
>> +	case BRBINF_EL_EL1:
>> +		return PERF_BR_KERNEL;
>> +	case BRBINF_EL_EL2:
>> +		if (is_kernel_in_hyp_mode())
>> +			return PERF_BR_KERNEL;
>> +		return PERF_BR_HV;
>> +	default:
>> +		pr_warn("unknown branch privilege captured\n");
>> +		return -1;
>> +	}
>> +}
>> +
>>  static void capture_brbe_flags(struct pmu_hw_events *cpuc, struct perf_event *event,
>>  			       u64 brbinf, int idx)
>>  {
>> @@ -302,6 +321,15 @@ static void capture_brbe_flags(struct pmu_hw_events *cpuc, struct perf_event *ev
>>  			cpuc->brbe_entries[idx].in_tx = brbinf & BRBINF_TX;
>>  		}
>>  	}
>> +
>> +	if (branch_sample_priv(event)) {
>> +		/*
>> +		 * All these information (i.e branch privilege level) are not
>> +		 * available for source only branch records.
>> +		 */
>> +		if (type != BRBINF_VALID_SOURCE)
>> +			cpuc->brbe_entries[idx].priv = brbe_fetch_perf_priv(brbinf);
>> +	}
>>  }
>>  
>>  /*
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index 916ce5102b33..8021b6a30d86 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -1688,4 +1688,9 @@ static inline bool branch_sample_hw_index(const struct perf_event *event)
>>  {
>>  	return event->attr.branch_sample_type & PERF_SAMPLE_BRANCH_HW_INDEX;
>>  }
>> +
>> +static inline bool branch_sample_priv(const struct perf_event *event)
>> +{
>> +	return event->attr.branch_sample_type & PERF_SAMPLE_BRANCH_PRIV_SAVE;
>> +}
>>  #endif /* _LINUX_PERF_EVENT_H */
>> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
>> index 361fdc6b87a0..4d77710f7a4e 100644
>> --- a/include/uapi/linux/perf_event.h
>> +++ b/include/uapi/linux/perf_event.h
>> @@ -204,6 +204,8 @@ enum perf_branch_sample_type_shift {
>>  
>>  	PERF_SAMPLE_BRANCH_HW_INDEX_SHIFT	= 17, /* save low level index of raw branch records */
>>  
>> +	PERF_SAMPLE_BRANCH_PRIV_SAVE_SHIFT	= 18, /* save privillege mode */
>> +
>>  	PERF_SAMPLE_BRANCH_MAX_SHIFT		/* non-ABI */
>>  };
>>  
>> @@ -233,6 +235,8 @@ enum perf_branch_sample_type {
>>  
>>  	PERF_SAMPLE_BRANCH_HW_INDEX	= 1U << PERF_SAMPLE_BRANCH_HW_INDEX_SHIFT,
>>  
>> +	PERF_SAMPLE_BRANCH_PRIV_SAVE	= 1U << PERF_SAMPLE_BRANCH_PRIV_SAVE_SHIFT,
>> +
>>  	PERF_SAMPLE_BRANCH_MAX		= 1U << PERF_SAMPLE_BRANCH_MAX_SHIFT,
>>  };
>>  
>> @@ -265,6 +269,12 @@ enum {
>>  	PERF_BR_MAX,
>>  };
>>  
>> +enum {
>> +	PERF_BR_USER	= 0,
>> +	PERF_BR_KERNEL	= 1,
>> +	PERF_BR_HV	= 2,
>> +};
>> +
> Can we have 0 as "UNKNOWN". It's going to be difficult to parse files when privilege information
> isn't saved and get accurate results without that. For example if it's not set then presumably
> the field would be 0 (PERF_BR_USER), but that doesn't mean the samples are user in that case.
> 
> I know you might be able to go backwards and look at what arguments were passed to the kernel but
> it's not guaranteed that the kernel honored the request anyway. There are also other platforms
> to think about etc.
> 
> If you look at the branch type definitions above they start at 0 (PERF_BR_UNKNOWN) which I think
> works out quite nicely in the userspace code.

This is being taken care in the new BRBE related perf ABI changes series (V3).

https://lore.kernel.org/all/20220314055857.125421-1-anshuman.khandual@arm.com/