Re: [PATCH V2 4/8] perf: Capture branch privilege information

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

On 3/10/22 15:51, James Clark wrote:
> 
> 
> On 09/03/2022 03:36, 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 based flag PERF_SAMPLE_BRANCH_PRIV_SAVE.
>> This flag helps user choose whether privilege information is captured.
>>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
>> Cc: Jiri Olsa <jolsa@redhat.com>
>> Cc: Namhyung Kim <namhyung@kernel.org>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Will Deacon <will@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>
>> ---
>>  include/uapi/linux/perf_event.h | 13 ++++++++++++-
>>  1 file changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
>> index d29280adc3c4..0e96e2017f68 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,
>>  };
>>  
>> @@ -271,6 +275,12 @@ enum {
>>  	PERF_BR_NEW_MAX,
>>  };
>>  
>> +enum {
>> +	PERF_BR_USER	= 0,
>> +	PERF_BR_KERNEL	= 1,
>> +	PERF_BR_HV	= 2,
>> +};
> 
> 0 should be "PERF_BR_PRIV_UNKNOWN" so userspace knows if it was not enabled
> otherwise it will look like all samples are PERF_BR_USER when actually
> priv type recording was just disabled.

Makes sense.

> 
> I think it's not even always possible to go backwards from a sample to
> work out what the event attributes were so this can be interpreted (taking
> all of perf script and every corner case into account).
> 
> Starting at 0=UNKNOWN is consistent with the other fields and makes parsing
> it a whole lot easier.

Fair enough, will start the enum with PERF_BR_PRIV_UNKNOWN and expand the
field 'perf_branch_entry.priv' to 3 bits instead, in order to accommodate
any future needs regarding other captured privilege levels.

> 
> James
> 
>> +
>>  #define PERF_SAMPLE_BRANCH_PLM_ALL \
>>  	(PERF_SAMPLE_BRANCH_USER|\
>>  	 PERF_SAMPLE_BRANCH_KERNEL|\
>> @@ -1386,7 +1396,8 @@ struct perf_branch_entry {
>>  		cycles:16,  /* cycle count to last branch */
>>  		type:4,     /* branch type */
>>  		new_type:4, /* additional branch type */
>> -		reserved:36;
>> +		priv:2,     /* privilege level */
>> +		reserved:34;
>>  };
>>  
>>  union perf_sample_weight {