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

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Message-ID: <YOEFGcTJC6AWFgs1@Ryzen-9-3900X.localdomain>
Date:   Sat, 3 Jul 2021 17:47:21 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Fangrui Song <maskray@...gle.com>,
        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 02, 2021 at 11:29:31AM -0700, Nick Desaulniers wrote:
> 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?

Is there any reason we cannot just add this sort of logic to the main
Makefile?

Such as (indentation to emphasis diff):

ifeq ($(CROSS_COMPILE),)
ifneq ($(LLVM),)
ifeq ($(LLVM_IAS),1)
	ifeq ($(ARCH),arm64)
		TENTATIVE_CLANG_FLAGS	+= --target=aarch64-linux
	else ifeq ($(ARCH),s390)
		TENTATIVE_CLANG_FLAGS	+= --target=s390x-linux
	else ifeq ($(ARCH),x86_64)
		TENTATIVE_CLANG_FLAGS	+= --target=x86_64-linux
	else
		$(error Specify CROSS_COMPILE or add '--target=' option to Makefile)
	endif
endif
endif
else
TENTATIVE_CLANG_FLAGS	+= --target=$(notdir $(CROSS_COMPILE:%-=%))
ifeq ($(LLVM_IAS),1)
TENTATIVE_CLANG_FLAGS	+= -integrated-as
else
TENTATIVE_CLANG_FLAGS	+= -no-integrated-as
GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
TENTATIVE_CLANG_FLAGS	+= --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
endif
endif

I know this looks a little cumbersome but it does help us avoid
duplication across architecture Makefiles and ordering dependencies.

Cheers,
Nathan