linux-kernel - Re: [PATCH] kbuild: reuse vmlinux.o in vmlinux

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Message-ID: <CAK7LNAQ1HTTSq-yp0t6Z4V-Z9PHg+O2n=NC89FJJFNQnhaOcSA@mail.gmail.com>
Date:   Sat, 23 May 2020 03:16:49 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kbuild: reuse vmlinux.o in vmlinux_link

On Fri, May 22, 2020 at 7:08 AM Kees Cook <keescook@...omium.org> wrote:
>
> On Thu, May 21, 2020 at 01:27:16PM -0700, Sami Tolvanen wrote:
> > Instead of linking all compilation units again each time vmlinux_link is
> > called, reuse vmlinux.o from modpost_link.
> >
> > With x86_64 allyesconfig, vmlinux_link is called three times and reusing
> > vmlinux.o reduces the build time ~38 seconds on my system (59% reduction
> > in the time spent in vmlinux_link).
>
> Nice! Any time savings at final link is a big cumulative win.
>
> > Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> > ---
> >  scripts/link-vmlinux.sh | 5 +----
> >  1 file changed, 1 insertion(+), 4 deletions(-)
> >
> > diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> > index d09ab4afbda4..c6cc4305950c 100755
> > --- a/scripts/link-vmlinux.sh
> > +++ b/scripts/link-vmlinux.sh
> > @@ -77,11 +77,8 @@ vmlinux_link()
> >
> >       if [ "${SRCARCH}" != "um" ]; then
> >               objects="--whole-archive                        \
> > -                     ${KBUILD_VMLINUX_OBJS}                  \
> > +                     vmlinux.o                               \
> >                       --no-whole-archive                      \
> > -                     --start-group                           \
> > -                     ${KBUILD_VMLINUX_LIBS}                  \
> > -                     --end-group                             \
> >                       ${@}"
> >
> >               ${LD} ${KBUILD_LDFLAGS} ${LDFLAGS_vmlinux}      \
>
> I think the "um" case can be updated as well too, yes?

I agree.

I changed the um part, then ARCH=um build is successful.







> Also, I think the comment above modpost_link() needs to be updated now
> to reflect the nature of how vmlinux.o gets used after this patch.
>
> --
> Kees Cook



-- 
Best Regards
Masahiro Yamada