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From:   Vince Weaver <vincent.weaver@maine.edu>
Date:   Fri, 24 Aug 2018 17:09:27 -0400 (EDT)
To:     Peter Zijlstra <peterz@infradead.org>
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
In-Reply-To: <20180824085042.GL24124@hirez.programming.kicks-ass.net>
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References: <alpine.DEB.2.21.1808231401440.28868@macbook-air> <20180824085042.GL24124@hirez.programming.kicks-ass.net>
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On Fri, 24 Aug 2018, Peter Zijlstra wrote:

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

Well, it should probably be documented either in the manpage or else in 
perf_event.h  (even if it's just "internal use, don't use") as we can't 
really expect people to download a git tree and do a git-blame to try to 
figure out what this mysterious field is all about.

Also, this change increased the size of the enum from 32 to 64 bits on 
32-bit machines, though that only really matters if the user is doing 
something really weird with enum variables.

Vince