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

Peter Zijlstra [off-list ref] writes:

quoted

On Tue, Apr 11, 2017 at 07:21:05AM +0530, Madhavan Srinivasan wrote:

quoted

From: Sukadev Bhattiprolu <redacted>

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.

Actually that's only partly true. As I describe above the PERF_MEM_NA
value is currently exported on BE platforms when a user requests it.

So I added this text after the output from perf_mem__tlb_scnprintf():

   Because of the way the perf tool code is written this is still displayed to the
   user as "N/A", so there is no bug visible at the UI level.
   
   Currently there are no big endian architectures which export a meaningful
   value (ie. other than PERF_MEM_NA), so the extent of the bug on big endian
   platforms is that the PERF_MEM_NA value is exported incorrectly as described
   above. Subsequent patches will add support on big endian powerpc for populating
   the data source value.


Hope that is clear.

It also occurred to me that we don't actually have to redefine the whole
union, it's only the bitfields that matter, so we could reduce the diff
to:

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index c66a485a24ac..97152c79df6b 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h

@@ -894,12 +894,23 @@ enum perf_callchain_context {
  union perf_mem_data_src {
  	__u64 val;
  	struct {
+#if defined(__LITTLE_ENDIAN_BITFIELD)
  		__u64   mem_op:5,	/* type of opcode */
  			mem_lvl:14,	/* memory hierarchy level */
  			mem_snoop:5,	/* snoop mode */
  			mem_lock:2,	/* lock instr */
  			mem_dtlb:7,	/* tlb access */
  			mem_rsvd:31;
+#elif defined(__BIG_ENDIAN_BITFIELD)
+		__u64	mem_rsvd:31,
+			mem_dtlb:7,	/* tlb access */
+			mem_lock:2,	/* lock instr */
+			mem_snoop:5,	/* snoop mode */
+			mem_lvl:14,	/* memory hierarchy level */
+			mem_op:5;	/* type of opcode */
+#else
+#error "Unknown endianness"
+#endif
  	};
  };

That looks better to me, thoughts?