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

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Date:   Thu, 13 Apr 2017 14:38:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>
Cc:     mpe@...erman.id.au, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, benh@...nel.crashing.org,
        paulus@...ba.org, sukadev@...ux.vnet.ibm.com,
        andrew.donnellan@....ibm.com, mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, wangnan0@...wei.com,
        ast@...nel.org, eranian@...gle.com
Subject: Re: [PATCH v3 1/6] powerpc/perf: Define big-endian version of
 perf_mem_data_src

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

Aside from that amendment,

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>