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

[Masahiro Yamada <masahiroy@kernel.org>]

On Mon, Oct 23, 2023 at 12:19 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Sun, Oct 22, 2023 at 1:24 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
> >
> > On Sun, Oct 22, 2023 at 4:33 AM Andrii Nakryiko
> > <andrii.nakryiko@gmail.com> wrote:
> > >
> > > On Sat, Oct 21, 2023 at 4:38 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
> > > >
> > > > 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.
> > > >
> > >
> > > Could you please also prioritize not regressing informativeness of a
> > > build log? With your changes it's not clear now if BTF was generated
> > > or not for a kernel module, while previously it was obvious and was
> > > easy to spot if for some reason BTF was not generated. I'd like to
> > > preserve this
> > > property, thank you.
> > >
> > > E.g, can we still have BTF generation as a separate command and do a
> > > separate $(call if_changed,btf_ko)? Or something along those lines.
> > > Would that work?
> >
> > If we have an intermediate file (say, *.no-btf.ko),
> > it would make sense to have separate
> > $(call if_changed,ld_ko_o) and $(call if_changed,btf_ko).
>
> Currently we don't generate intermediate files, but we do rewrite
> original .ko file as a post-processing step.
>
> And that rewriting step might not happen depending on Kconfig and
> toolchain (e.g., too old pahole makes it impossible to generate kernel
> module BTF). And that's why having a separate BTF [M] message in the
> build log is important.
>
> >
> >
> >            LD                 RESOLVE_BTFIDS
> >  *.mod.o  ------> *.no-btf.ko ------------> *.ko
> >
> >
> > When vmlinux is changed, only the second step would
> > be re-run, but that would require extra file copy.
>
> Today we rewrite .ko with a new .ko ELF file which gains a new ELF
> section (.BTF), so we already pay this price when BTF is enabled (if
> that's your concern).
>
> >
> > Is this what you want to see?
>
> I don't have strong preferences for exact implementation, but what you
> propose will work, I think. What I'd like to avoid is unnecessarily
> relinking .ko files if all we need to do is regenerate BTF.




Is there any way to make pahole/resolve_btfids
take separate input and output files
instead of in-place modification?

Otherwise, explicit "cp *.no-btf.ko *.ko" would be needed.





--
Best Regards
Masahiro Yamada