Re: [bpf-next PATCH v2 4/4] kbuild: refactor module BTF rule

From: Masahiro Yamada <masahiroy@kernel.org>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Jiri Olsa <olsajiri@gmail.com>,
	linux-kbuild@vger.kernel.org,  linux-kernel@vger.kernel.org,
	Nathan Chancellor <nathan@kernel.org>,
	 Nick Desaulniers <ndesaulniers@google.com>,
	Nicolas Schier <nicolas@fjasle.eu>,
	bpf@vger.kernel.org
Subject: Re: [bpf-next PATCH v2 4/4] kbuild: refactor module BTF rule
Date: Sat, 21 Oct 2023 20:37:24 +0900	[thread overview]
Message-ID: <CAK7LNAQxFgOpuCBYPSx5Z6aw5MtKzPL39XLUvZuUBSyRGnOZUg@mail.gmail.com> (raw)
In-Reply-To: <CAEf4BzbYwEFSNTFjJyhYmOOK5iwHjFAdcArkUbcQz5ntRvOOvA@mail.gmail.com>

On Sat, Oct 21, 2023 at 5:52 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Fri, Oct 20, 2023 at 12:03 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
> >
> > On Fri, Oct 20, 2023 at 7:55 AM Andrii Nakryiko
> > <andrii.nakryiko@gmail.com> wrote:
> > >
> > > On Thu, Oct 19, 2023 at 1:15 AM Jiri Olsa <olsajiri@gmail.com> wrote:
> > > >
> > > > On Thu, Oct 19, 2023 at 12:19:50AM +0900, Masahiro Yamada wrote:
> > > > > newer_prereqs_except and if_changed_except are ugly hacks of the
> > > > > newer-prereqs and if_changed in scripts/Kbuild.include.
> > > > >
> > > > > Remove.
> > > > >
> > > > > Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> > > > > ---
> > > > >
> > > > > Changes in v2:
> > > > >   - Fix if_changed_except to if_changed
> > > > >
> > > > >  scripts/Makefile.modfinal | 25 ++++++-------------------
> > > > >  1 file changed, 6 insertions(+), 19 deletions(-)
> > > > >
> > > > > diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
> > > > > index 9fd7a26e4fe9..fc07854bb7b9 100644
> > > > > --- a/scripts/Makefile.modfinal
> > > > > +++ b/scripts/Makefile.modfinal
> > > > > @@ -19,6 +19,9 @@ vmlinux :=
> > > > >  ifdef CONFIG_DEBUG_INFO_BTF_MODULES
> > > > >  ifneq ($(wildcard vmlinux),)
> > > > >  vmlinux := vmlinux
> > > > > +cmd_btf = ; \
> > > > > +     LLVM_OBJCOPY="$(OBJCOPY)" $(PAHOLE) -J $(PAHOLE_FLAGS) --btf_base vmlinux $@; \
> > > > > +     $(RESOLVE_BTFIDS) -b vmlinux $@
> > > > >  else
> > > > >  $(warning Skipping BTF generation due to unavailability of vmlinux)
> > > > >  endif
> > > > > @@ -41,27 +44,11 @@ quiet_cmd_ld_ko_o = LD [M]  $@
> > > > >        cmd_ld_ko_o +=                                                 \
> > > > >       $(LD) -r $(KBUILD_LDFLAGS)                                      \
> > > > >               $(KBUILD_LDFLAGS_MODULE) $(LDFLAGS_MODULE)              \
> > > > > -             -T scripts/module.lds -o $@ $(filter %.o, $^)
> > > > > +             -T scripts/module.lds -o $@ $(filter %.o, $^)           \
> > > > > +     $(cmd_btf)
> > > > >
> > > > > -quiet_cmd_btf_ko = BTF [M] $@
> > > >
> > > > nit not sure it's intentional but we no longer display 'BTF [M] ...ko' lines,
> > > > I don't mind not displaying that, but we should mention that in changelog
> > > >
> > >
> > > Thanks for spotting this! I think those messages are useful and
> > > important to keep. Masahiro, is it possible to preserve them?
> >
> >
> >
> > No, I do not think so.
> >
>
> That's too bad, I think it's a useful one.

I prioritize that the code is correct.

>
> > Your code is wrong.
> >
>
> Could be, but note the comment you are removing:
>
> # Re-generate module BTFs if either module's .ko or vmlinux changed
>
> BTF has to be re-generated not just when module .ko is regenerated,
> but also when the vmlinux image itself changes.
>
> I don't see where this is done with your changes. Can you please point
> it out explicitly?

That is too obvious; %.ko depends on $(vmlinux).

%.ko: %.o %.mod.o scripts/module.lds $(vmlinux) FORCE

-- 
Best Regards
Masahiro Yamada