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

[Madhavan Srinivasan <maddy@linux.vnet.ibm.com>]

On Thursday 13 April 2017 06:53 PM, Michael Ellerman wrote:
> 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.

Thanks mpe.

Maddy

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