linux-kernel - Re: [PATCH 2/2] MIPS: VDSO: Do not disable VDSO when linking with ld.lld

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Date:   Sun, 19 Apr 2020 13:05:58 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Dmitry Golovin <dima@...ovin.in>,
        Sedat Dilek <sedat.dilek@...il.com>
Subject: Re: [PATCH 2/2] MIPS: VDSO: Do not disable VDSO when linking with
 ld.lld

Hi Masahiro,

On Mon, Apr 20, 2020 at 04:32:20AM +0900, Masahiro Yamada wrote:
> On Mon, Apr 20, 2020 at 3:17 AM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > On Sun, Apr 19, 2020 at 11:04:45AM -0700, Nathan Chancellor wrote:
> > > Currently, when linking with ld.lld, this warning pops up:
> > >
> > >     arch/mips/vdso/Makefile:70: MIPS VDSO requires binutils >= 2.25
> > >
> > > ld-ifversion calls ld-version, which calls scripts/ld-version.sh, which
> > > is specific to GNU ld. ld.lld has a completely different versioning
> > > scheme (as it follows LLVM's versioning) and it does not have the issue
> > > mentioned in the comment above this block so it should not be subjected
> > > to this check.
> > >
> > > With this patch, the VDSO successfully links and shows P_MIPS_PC32 in
> > > vgettimeofday.o.
> > >
> > > $ llvm-objdump -Dr arch/mips/vdso/vgettimeofday.o | grep R_MIPS_PC32
> > >                       00000024:  R_MIPS_PC32  _start
> > >                       000000b0:  R_MIPS_PC32  _start
> > >                       000002bc:  R_MIPS_PC32  _start
> > >                       0000036c:  R_MIPS_PC32  _start
> > >                       00000468:  R_MIPS_PC32  _start
> > >
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/785
> > > Link: https://github.com/llvm/llvm-project/commit/e364e2e9ce50c12eb2bf093560e1a1a8544d455a
> > > Reported-by: Dmitry Golovin <dima@...ovin.in>
> > > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> > > ---
> > >  arch/mips/vdso/Makefile | 8 +++++---
> > >  1 file changed, 5 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/mips/vdso/Makefile b/arch/mips/vdso/Makefile
> > > index d7fe8408603e..f99e583d14a1 100644
> > > --- a/arch/mips/vdso/Makefile
> > > +++ b/arch/mips/vdso/Makefile
> > > @@ -65,9 +65,11 @@ DISABLE_VDSO := n
> > >  # the comments on that file.
> > >  #
> > >  ifndef CONFIG_CPU_MIPSR6
> > > -  ifeq ($(call ld-ifversion, -lt, 225000000, y),y)
> > > -    $(warning MIPS VDSO requires binutils >= 2.25)
> > > -    DISABLE_VDSO := y
> > > +  ifndef CONFIG_LD_IS_LLD
> > > +    ifeq ($(call ld-ifversion, -lt, 225000000, y),y)
> > > +      $(warning MIPS VDSO requires binutils >= 2.25)
> > > +      DISABLE_VDSO := y
> > > +    endif
> > >    endif
> > >  endif
> > >
> > > --
> > > 2.26.1
> > >
> >
> > Hmmm, I still see this warning when first runing make <config>... I
> > assume because this Makefile gets parsed before Kconfig runs.
> 
> 
> I do not see the warning for 'make <config>'.
> 
> Could you tell me how to reproduce it?
> 
> 
> For cleaning, indeed, i see the warning.
> 
> 
> $ make ARCH=mips  LLVM=1 clean
> arch/mips/vdso/Makefile:70: MIPS VDSO requires binutils >= 2.25

This is enough. I think I figured out how to avoid it, I'll be sending
out v2 shortly, I'd appreciate any comments that you have.

> >
> > Perhaps it would be better to check if ld-version is 0 (since that means
> > we are not using GNU ld):
> >
> > ifneq ($(call ld-ifversion, -eq, 0, y),y)
> >
> > I am open to suggestions though.
> >
> > Cheers,
> > Nathan
> >
> > --
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> > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20200419181715.GA36234%40ubuntu-s3-xlarge-x86.
> 
> 
> 
> -- 
> Best Regards
> Masahiro Yamada
> 

Cheers,
Nathan