[perf] perf_event.h ABI visibility question

[Vince Weaver]

I notice that Linux 4.18 has the following changeset which changes the
user visible perf_event.h file

	commit 6cbc304f2f360f25cc8607817239d6f4a2fd3dc5
	Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
	Date:   Thu May 10 15:48:41 2018 +0200

    perf/x86/intel: Fix unwind errors from PEBS entries (mk-II)

which contains

--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -143,6 +143,8 @@ enum perf_event_sample_format {
        PERF_SAMPLE_PHYS_ADDR                   = 1U << 19,
 
        PERF_SAMPLE_MAX = 1U << 20,             /* non-ABI */
+
+       __PERF_SAMPLE_CALLCHAIN_EARLY           = 1ULL << 63,
 };


Is this supposed to be a user-visible interface?

I realize that if the user tries to set anything above PERF_SAMPLE_MAX
it will be caught and flagged as EINVAL.

However even with the double-underscore hint in 
__PERF_SAMPLE_CALLCHAIN_EARLY the value is still in the user-visible 
header so it's now part of the ABI and I guess the manpage has to document it.

Vince