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

From: Peter Zijlstra
Date: Thu Apr 13 2017 - 08:39:19 EST


On Tue, Apr 11, 2017 at 07:21:05AM +0530, Madhavan Srinivasan wrote:
> From: Sukadev Bhattiprolu <sukadev@xxxxxxxxxxxxxxxxxx>
>
> 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.

Aside from that amendment,

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>