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

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Message-ID: <20210702112210.GA11084@willie-the-truck>
Date:   Fri, 2 Jul 2021 12:22:10 +0100
From:   Will Deacon <will@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Fangrui Song <maskray@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] arm64: drop CROSS_COMPILE for LLVM=1 LLVM_IAS=1

On Thu, Jul 01, 2021 at 04:55:05PM -0700, Nick Desaulniers wrote:
> We get constant feedback that the command line invocation of make is too
> long. CROSS_COMPILE is helpful when a toolchain has a prefix of the
> target triple, or is an absolute path outside of $PATH, but it's mostly
> redundant for a given ARCH.
> 
> If CROSS_COMPILE is not set, simply set --target=aarch64-linux for
> CLANG_FLAGS, KBUILD_CFLAGS, and KBUILD_AFLAGS.
> 
> Previously, we'd cross compile via:
> $ ARCH=arm64 CROSS_COMPILE=aarch64-linxu-gnu make LLVM=1 LLVM_IAS=1
> Now:
> $ ARCH=arm64 make LLVM=1 LLVM_IAS=1
> 
> We can drop gnu from the triple, but dropping linux from the triple
> produces different .config files for the above invocations for the
> defconfig target.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1399
> Suggested-by: Arnd Bergmann <arnd@...nel.org>
> Suggested-by: Fangrui Song <maskray@...gle.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
>  arch/arm64/Makefile | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 7bc37d0a1b68..016873fddcc3 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -34,6 +34,17 @@ $(warning LSE atomics not supported by binutils)
>    endif
>  endif
>  
> +ifneq ($(LLVM),)
> +ifneq ($(LLVM_IAS),)
> +ifeq ($(CROSS_COMPILE),)
> +CLANG_TARGET	:=--target=aarch64-linux
> +CLANG_FLAGS	+= $(CLANG_TARGET)
> +KBUILD_CFLAGS	+= $(CLANG_TARGET)
> +KBUILD_AFLAGS	+= $(CLANG_TARGET)

Do we need to do anything extra for the linker here? I can't see how we
avoid picking up the host copy.

> +endif
> +endif
> +endif

Have you tested the compat vDSO with this change? I think we'll just end
up passing two --target options, which is hopefully ok, but thought I'd
better check.

Will