Re: [PATCH v3 1/6] powerpc/perf: Define big-endian version of perf_mem_data_src

[Michael Ellerman <mpe@ellerman.id.au>]

Peter Zijlstra <peterz@infradead.org> writes:

> On Tue, Apr 11, 2017 at 07:21:05AM +0530, Madhavan Srinivasan wrote:
>> From: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
>> 
>> perf_mem_data_src is an union that is initialized via the ->val field
>> and accessed via the bitmap fields. For this to work on big endian
>> platforms (Which is broken now), we also need a big-endian represenation
>> of perf_mem_data_src. i.e, in a big endian system, if user request
>> PERF_SAMPLE_DATA_SRC (perf report -d), will get the default value from
>> perf_sample_data_init(), which is PERF_MEM_NA. Value for PERF_MEM_NA
>> is constructed using shifts:
>> 
>>   /* TLB access */
>>   #define PERF_MEM_TLB_NA		0x01 /* not available */
>>   ...
>>   #define PERF_MEM_TLB_SHIFT	26
>> 
>>   #define PERF_MEM_S(a, s) \
>> 	(((__u64)PERF_MEM_##a##_##s) << PERF_MEM_##a##_SHIFT)
>> 
>>   #define PERF_MEM_NA (PERF_MEM_S(OP, NA)   |\
>> 		    PERF_MEM_S(LVL, NA)   |\
>> 		    PERF_MEM_S(SNOOP, NA) |\
>> 		    PERF_MEM_S(LOCK, NA)  |\
>> 		    PERF_MEM_S(TLB, NA))
>> 
>> Which works out as:
>> 
>>   ((0x01 << 0) | (0x01 << 5) | (0x01 << 19) | (0x01 << 24) | (0x01 << 26))
>> 
>> Which means the PERF_MEM_NA value comes out of the kernel as 0x5080021
>> in CPU endian.
>> 
>> But then in the perf tool, the code uses the bitfields to inspect the
>> value, and currently the bitfields are defined using little endian
>> ordering.
>> 
>> So eg. in perf_mem__tlb_scnprintf() we see:
>>   data_src->val = 0x5080021
>>              op = 0x0
>>             lvl = 0x0
>>           snoop = 0x0
>>            lock = 0x0
>>            dtlb = 0x0
>>            rsvd = 0x5080021
>> 
>> Patch does a minimal fix of adding big endian definition of the bitfields
>> to match the values that are already exported by the kernel on big endian.
>> And it makes no change on little endian.
>
> I think it is important to note that there are no current big-endian
> users. So 'fixing' this will not break anybody and will ensure future
> users (next patch) will work correctly.

Sure I'll fold in something along those lines.

> Aside from that amendment,
>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Thanks.

cheers