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

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Message-ID: <CAEf4BzbYwEFSNTFjJyhYmOOK5iwHjFAdcArkUbcQz5ntRvOOvA@mail.gmail.com>
Date:   Fri, 20 Oct 2023 13:51:54 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Jiri Olsa <olsajiri@...il.com>, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nicolas Schier <nicolas@...sle.eu>, bpf@...r.kernel.org
Subject: Re: [bpf-next PATCH v2 4/4] kbuild: refactor module BTF rule

On Fri, Oct 20, 2023 at 12:03 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> On Fri, Oct 20, 2023 at 7:55 AM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Thu, Oct 19, 2023 at 1:15 AM Jiri Olsa <olsajiri@...il.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@...nel.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.

> 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?

>
> To clarify this is a fix,
> I will replace the commit as follows:
>
>
>
>
> ------------------->8----------------------
> kbuild: detect btf command change for modules
>
> Currently, the command change in cmd_btf_ko does not cause to rebuild
> the modules because it is not passed to if_changed.
>
> Pass everything to if_change so that the btf command is also recorded
> in the .*.cmd files. This removes the hacky newer_prereqs_except and
> if_changed_except macros too.
> ------------------->8----------------------
>
>
>
>
> --
> Best Regards
>
> Masahiro Yamada