linux-kernel - Re: [PATCH 1/2] kbuild: Remove '-mno-global-merge'

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Message-ID: <CA+icZUV-pHn0mWCnHr2x9SrcbaQos=M=Eg7p2KOaca829DmN_w@mail.gmail.com>
Date:   Thu, 31 Mar 2022 20:52:55 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        Kees Cook <keescook@...omium.org>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-um@...ts.infradead.org, llvm@...ts.linux.dev,
        patches@...ts.linux.dev
Subject: Re: [PATCH 1/2] kbuild: Remove '-mno-global-merge'

On Thu, Mar 31, 2022 at 5:37 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> On Thu, Mar 31, 2022 at 09:11:12AM +0200, Sedat Dilek wrote:
> > On Thu, Mar 31, 2022 at 5:27 AM Nathan Chancellor <nathan@...nel.org> wrote:
> > >
> > > This flag is specific to clang, where it is only used by the 32-bit and
> > > 64-bit ARM backends. In certain situations, the presence of this flag
> > > will cause a warning, as shown by commit 6580c5c18fb3 ("um: clang: Strip
> > > out -mno-global-merge from USER_CFLAGS").
> > >
> > > Since commit 61163efae020 ("kbuild: LLVMLinux: Add Kbuild support for
> > > building kernel with Clang") that added this flag back in 2014, there
> > > have been quite a few changes to the GlobalMerge pass in LLVM. Building
> > > several different ARCH=arm and ARCH=arm64 configurations with LLVM 11
> > > (minimum) and 15 (current main version) with this flag removed (i.e.,
> > > with the default of '-mglobal-merge') reveals no modpost warnings, so it
> > > is likely that the issue noted in the comment is no longer relevant due
> > > to changes in LLVM or modpost, meaning this flag can be removed.
> > >
> > > If any new warnings show up that are a result of the removal of this
> > > flag, it can be added back under arch/arm{,64}/Makefile to avoid
> > > warnings on other architectures.
> > >
> > > Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> > > ---
> > >  Makefile | 4 ----
> > >  1 file changed, 4 deletions(-)
> > >
> > > diff --git a/Makefile b/Makefile
> > > index daeb5c88b50b..f2723d9bfca4 100644
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -784,10 +784,6 @@ ifdef CONFIG_CC_IS_CLANG
> > >  KBUILD_CPPFLAGS += -Qunused-arguments
> > >  # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable.
> > >  KBUILD_CFLAGS += -Wno-gnu
> > > -# CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the
> > > -# source of a reference will be _MergedGlobals and not on of the whitelisted names.
> > > -# See modpost pattern 2
> > > -KBUILD_CFLAGS += -mno-global-merge
> > >  else
> > >
> > >  # gcc inanely warns about local variables called 'main'
> > > --
> > > 2.35.1
> > >
> >
> > I have tested this several times and was able to boot into bar metal -
> > no problems with building and/or booting my kernel-modules.
> >
> > Tested-by: Sedat Dilek <sedat.dilek@...il.com>
> > Reviewed-by: Sedat Dilek <sedat.dilek@...il.com>
>
> I would be very concerned if you did see any impact, given this flag is
> ARM specific :) thanks as always for verifying!
>
> > Just as a side-note:
> > As with Linux v5.18-rc1 and -std=gnu11 we change the above comment ...?
> >
> > # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable.
> > KBUILD_CFLAGS += -Wno-gnu
>
> It was updated as part of the shift to '-std=gnu11':
>
> https://git.kernel.org/linus/e8c07082a810fbb9db303a2b66b66b8d7e588b53
>
> The UML tree is based on 5.17-rc6, which does not have that change.
> There should not be a merge conflict though.
>

Ah OK, a different base for UML.

- Sedat -