Re: [PATCH 2/2] pahole: Allow asking for extra features using the '+' prefix in --btf_features

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Mon, Apr 29, 2024 at 12:16:18PM +0100, Alan Maguire wrote:
> On 26/04/2024 21:47, Arnaldo Carvalho de Melo wrote:
> > On Fri, Apr 26, 2024 at 01:26:40PM -0700, Andrii Nakryiko wrote:
> >> On Fri, Apr 19, 2024 at 1:58 PM Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
> >> for older paholes that don't yet know about + syntax, but support
> >> --btf_features, will this effectively disable all features or how will
> >> it work?
> >>
> >> I'm thinking from the perspective of using +reproducible_build
> >> unconditionally in kernel's build scripts, will it regress something
> >> for current pahole versions?
> > 
> > Well, I think it will end up being discarded just like "all" or
> > "default", no? I.e. those were keywords not grokked by older pahole
> > versions, so ignored as we're not using --btf_features_strict, right?

> > Alan?
 
> Yep, it would just be ignored, so wouldn't have the desired behaviour
> of enabling defaults + reproducible build option.
 
> > But then we're not yet using --btf_features in scripts/Makefile.btf,
> > right?

> > But as Daniel pointed out and Alan (I think) agreed, for things like
> > scripts we probably end up using the most verbose thing as:

> > 	--btf_features=default,reproducible_build

> > to mean a set (the default set of BTF options) + an optional/extra
> > feature (reproducibe_build), that for people not used to the + syntax
> > may be more descriptive (I really think that both are confusing for
> > beginners knowing nothing about BTF and its evolution, etc).

> > Alan, also we released 1.26 with "all" meaning what we now call
> > "default", so we need to keep both meaning the same thing, right?

> I might be missing something here, but I think we should always call out

No you're not, I was just talking about what Daniel pointed out, that
when using a script using 'default,extra_feature' is more descriptive
than '+extra_feature', but you're right, for the kernel we go on adding
the features we need and older pahole versions will just ignore those.

The explanation you gave on this message is interesting to clarify when
'default' should be used and perhaps also use the kernel build process
needs/use of a list of features, etc. It would be good to have it in the
man page, can you provide a formal patch doing that?

Thanks,

- Arnaldo

> explicitly the set of features we want in the kernel Makefile.btf
> (something like [1]). The reason for this is that the concept of what is
> "default" may evolve over time; for example it's going to include
> Daniel's kfunc definitions for soon. That's a good thing, but it could
> conceivably cause problems down the line. Consider a newer pahole - with
> a newer set of defaults - running on an older kernel. In that case, we
> could end up encoding BTF features we don't want.  By contrast, if we
> always call out the full set of BTF features we want via
> --btf_features=feature1,feature2 etc we'll always get the expected set.
> Plus for folks consulting the code, it's much clearer which BTF features
> are in use when they look at the Makefiles for a particular kernel.
> So my sense of the value of "default" is as a shortcut for testing the
> latest and greatest set of BTF feature encoding, but not for use in the
> kernel tree Makefiles. Thanks!
> 
> Alan
> 
> [1]
> https://lore.kernel.org/bpf/20240424154806.3417662-7-alan.maguire@oracle.com/
> 
> > - Arnaldo
> >  
> >>> In the future we may want the '-' counterpart as a way to _remove_ some
> >>> of the standard set of BTF features.
> >>>
> >>> Cc: Alan Maguire <alan.maguire@oracle.com>
> >>> Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> >>> Cc: Daniel Xu <dxu@dxuuu.xyz>
> >>> Cc: Eduard Zingerman <eddyz87@gmail.com>
> >>> Cc: Jiri Olsa <jolsa@kernel.org>
> >>> Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
> >>> ---
> >>>  man-pages/pahole.1          | 6 ++++++
> >>>  pahole.c                    | 6 ++++++
> >>>  tests/reproducible_build.sh | 2 +-
> >>>  3 files changed, 13 insertions(+), 1 deletion(-)
> >>>
> >>
> >> [...]