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

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Date:   Mon, 27 Aug 2018 09:52:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vince Weaver <vincent.weaver@...ne.edu>
Cc:     linux-kernel@...r.kernel.org, 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: Re: [perf] perf_event.h ABI visibility question

On Fri, Aug 24, 2018 at 05:09:27PM -0400, Vince Weaver wrote:
> 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.


Something like so then?

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index eeb787b1c53c..f35eb72739c0 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -144,7 +144,7 @@ enum perf_event_sample_format {
 
 	PERF_SAMPLE_MAX = 1U << 20,		/* non-ABI */
 
-	__PERF_SAMPLE_CALLCHAIN_EARLY		= 1ULL << 63,
+	__PERF_SAMPLE_CALLCHAIN_EARLY		= 1ULL << 63, /* non-ABI; internal use */
 };
 
 /*

> 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.

Yeah, since the target variable is a u64 I really can't be bothered with
that.