Re: [PATCH] riscv: Use -mno-relax when using lld linker

From: Nathan Chancellor <nathan@kernel.org>
To: Khem Raj <raj.khem@gmail.com>,
	linux-riscv@lists.infradead.org,
	clang-built-linux@googlegroups.com
Cc: Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH] riscv: Use -mno-relax when using lld linker
Date: Fri, 14 May 2021 14:14:37 -0700	[thread overview]
Message-ID: <bd8ae62f-1ead-2d82-5fdb-87308accc023@kernel.org> (raw)
In-Reply-To: <20210514205643.383422-1-raj.khem@gmail.com>

Hi Khem,

On 5/14/2021 1:56 PM, Khem Raj wrote:
> lld does not implement the RISCV relaxation optimizations like GNU ld
> therefore disable it when building with LLVM=1, Also pass it to
> assembler when using external GNU assembler ( LLVM_IAS != 1 ), this
> ensures that relevant assembler option is also enabled along. if these
> options are not used then we see following relocations in objects
> 
> 0000000000000000 R_RISCV_ALIGN     *ABS*+0x0000000000000002
> 
> These are then rejected by lld
> ld.lld: error: capability.c:(.fixup+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax but the .o is already compiled with -mno-relax
> 
> Signed-off-by: Khem Raj <raj.khem@gmail.com>
> Cc: Paul Walmsley <paul.walmsley@sifive.com>
> Cc: Palmer Dabbelt <palmer@dabbelt.com>
> Cc: Albert Ou <aou@eecs.berkeley.edu>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>

Thank you for the patch! I can build a ARCH=riscv defconfig kernel with 
LLVM=1 now. LLVM_IAS=1 still needs work but we have outstanding issues 
for that:

https://github.com/ClangBuiltLinux/linux/issues/1023
https://github.com/ClangBuiltLinux/linux/issues/1143

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

One comment below though.

> ---
>   arch/riscv/Makefile | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index 3eb9590a0775..519f133e0d53 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -38,6 +38,15 @@ else
>   	KBUILD_LDFLAGS += -melf32lriscv
>   endif
>   
> +ifeq ($(LLVM),1)

ifeq ($(CONFIG_LD_IS_LLD),y)

is a better choice so that LD=riscv64-linux-gnu-ld LLVM=1 can still work.

> +	KBUILD_CFLAGS += -mno-relax
> +	KBUILD_AFLAGS += -mno-relax
> +ifneq ($(LLVM_IAS),1)
> +	KBUILD_CFLAGS += -Wa,-mno-relax
> +	KBUILD_AFLAGS += -Wa,-mno-relax
> +endif
> +endif
> +
>   # ISA string setting
>   riscv-march-$(CONFIG_ARCH_RV32I)	:= rv32ima
>   riscv-march-$(CONFIG_ARCH_RV64I)	:= rv64ima
> 

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