linux-kernel - Re: [PATCH] arm64: drop CROSS_COMPILE for LLVM=1 LLVM

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Date:   Fri, 2 Jul 2021 11:29:31 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Fangrui Song <maskray@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] arm64: drop CROSS_COMPILE for LLVM=1 LLVM_IAS=1

On Fri, Jul 2, 2021 at 4:59 AM Arnd Bergmann <arnd@...nel.org> wrote:
>
> On Fri, Jul 2, 2021 at 1:55 AM 'Nick Desaulniers' via Clang Built
> Linux <clang-built-linux@...glegroups.com> wrote:
> >
> > +ifneq ($(LLVM),)
> > +ifneq ($(LLVM_IAS),)
> > +ifeq ($(CROSS_COMPILE),)
> > +CLANG_TARGET   :=--target=aarch64-linux
> > +CLANG_FLAGS    += $(CLANG_TARGET)
> > +KBUILD_CFLAGS  += $(CLANG_TARGET)
> > +KBUILD_AFLAGS  += $(CLANG_TARGET)
> > +endif
> > +endif
> > +endif
>
> I think only the "CLANG_TARGET   :=--target=aarch64-linux" line should
> go into the
> per-architecture Makefile. It doesn't hurt to just set that
> unconditionally here,
> and then change the CLANG_FLAGS logic in the top-level Makefile to use this
> in place of $(notdir $(CROSS_COMPILE:%-=%)).

I don't think we can do that. Based on the order the arch/ specific
Makefiles are included, if we don't eagerly add --target to the
KBUILD_{C|A}FLAGS, then cc-option, as-option, and as-instr macros
(defined in scripts/Makefile.compiler) checks in per arch/ Makefiles
may fail erroneously because --target was not set for
KBUILD_{C|A}FLAGS yet.

Another issue is the order of operations between the top level
Makefile and the per arch/ Makefiles.  The `notdir` block you
reference occurs earlier than the per-arch includes:

 609 TENTATIVE_CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%))
...
 648 include $(srctree)/arch/$(SRCARCH)/Makefile

We would need the opposite order to do what you describe. Reordering
these would effectively be a revert of
commit ae6b289a3789 ("kbuild: Set KBUILD_CFLAGS before incl. arch Makefile")
which I'm not sure we want to do.  But maybe there's another way I'm
not seeing yet?
-- 
Thanks,
~Nick Desaulniers