[perf] perf_event.h ABI visibility question

[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@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.

Vince

Re: [perf] perf_event.h ABI visibility question

[Peter Zijlstra]

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?

Re: [perf] perf_event.h ABI visibility question

[Vince Weaver]

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

Re: [perf] perf_event.h ABI visibility question

[Peter Zijlstra]

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.

Re: [perf] perf_event.h ABI visibility question

[Vince Weaver]

On Mon, 27 Aug 2018, Peter Zijlstra wrote:

> 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 */
>  };

yes, something like that would be fine.

I am being difficult about this, but from experience when trying to keep 
the manpage updated, what seems obvious now will not be so obvious 6 
months from now and trying to dig through the git logs / mailing list 
archives to verify the purpose of an addition can be a lot of work 
sometimes.

Vince

Re: [perf] perf_event.h ABI visibility question

[Peter Zijlstra]

On Tue, Aug 28, 2018 at 01:51:29PM -0400, Vince Weaver wrote:
> On Mon, 27 Aug 2018, Peter Zijlstra wrote:
> 
> > 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 */
> >  };
> 
> yes, something like that would be fine.
> 
> I am being difficult about this, but from experience when trying to keep 
> the manpage updated, what seems obvious now will not be so obvious 6 
> months from now and trying to dig through the git logs / mailing list 
> archives to verify the purpose of an addition can be a lot of work 
> sometimes.

Yeah, no worries. I appreciate you paying attention to these things.