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Date:   Fri, 24 Aug 2018 10:50:42 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Vince Weaver <vincent.weaver@maine.edu>
Cc:     linux-kernel@vger.kernel.org, Josh Poimboeuf <jpoimboe@redhat.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Stephane Eranian <eranian@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>
Subject: Re: [perf] perf_event.h ABI visibility question
Message-ID: <20180824085042.GL24124@hirez.programming.kicks-ass.net>
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On Thu, Aug 23, 2018 at 02:25:06PM -0400, Vince Weaver wrote:
> 
> 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@infradead.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.

Hurphm.. visible yes, but as you say, also quite useless. Does it really
make sense to document that?