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

[Madhavan Srinivasan]

On Thursday 13 April 2017 06:08 PM, Peter Zijlstra wrote:
> 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>

Thanks
Maddy