linux-kernel - [perf] perf_event.h ABI visibility question

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Date:   Thu, 23 Aug 2018 14:25:06 -0400 (EDT)
From:   Vince Weaver <vincent.weaver@...ne.edu>
To:     Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org
cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Stephane Eranian <eranian@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: [perf] perf_event.h ABI visibility question


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@...radead.org>
	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