linux-kernel - Re: [PATCH v2 5/5] LoongArch: Enable jump table with GCC for objtool

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Message-ID: <20241105141530.GE10375@noisy.programming.kicks-ass.net>
Date: Tue, 5 Nov 2024 15:15:30 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Huacai Chen <chenhuacai@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>, loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/5] LoongArch: Enable jump table with GCC for objtool

On Tue, Nov 05, 2024 at 08:39:06PM +0800, Tiezhu Yang wrote:
> For now, it is time to remove the compiler option -fno-jump-tables
> to enable jump table for objtool if the compiler is GCC and it has
> the compiler option -mannotate-tablejump, otherwise still keep the
> compiler option -fno-jump-tables to maintain compatibility with the
> older compilers.
> 
> By the way, the compiler behaviors are different for various archs,
> there are some corner issues after removing -fno-jump-tables if the
> compiler is Clang, so just keep the compiler option -fno-jump-tables
> for Clang at present.
> 
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> ---
>  arch/loongarch/Kconfig  | 3 +++
>  arch/loongarch/Makefile | 9 +++++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
> index bb35c34f86d2..500ee9b2cd88 100644
> --- a/arch/loongarch/Kconfig
> +++ b/arch/loongarch/Kconfig
> @@ -284,6 +284,9 @@ config AS_HAS_LBT_EXTENSION
>  config AS_HAS_LVZ_EXTENSION
>  	def_bool $(as-instr,hvcl 0)
>  
> +config CC_HAS_ANNOTATE_TABLEJUMP
> +	def_bool $(cc-option,-mannotate-tablejump)
> +
>  menu "Kernel type and options"
>  
>  source "kernel/Kconfig.hz"
> diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
> index ae3f80622f4c..61484df4eccc 100644
> --- a/arch/loongarch/Makefile
> +++ b/arch/loongarch/Makefile
> @@ -101,8 +101,17 @@ KBUILD_AFLAGS			+= $(call cc-option,-mthin-add-sub) $(call cc-option,-Wa$(comma)
>  KBUILD_CFLAGS			+= $(call cc-option,-mthin-add-sub) $(call cc-option,-Wa$(comma)-mthin-add-sub)
>  
>  ifdef CONFIG_OBJTOOL
> +ifdef CONFIG_CC_IS_GCC
> +ifdef CONFIG_CC_HAS_ANNOTATE_TABLEJUMP
> +KBUILD_CFLAGS			+= $(call cc-option,-mannotate-tablejump)
> +else
>  KBUILD_CFLAGS			+= -fno-jump-tables
>  endif
> +endif
> +ifdef CONFIG_CC_IS_CLANG
> +KBUILD_CFLAGS			+= -fno-jump-tables
> +endif
> +endif

This seems excessive. Why split between GCC and Clang, isn't
CC_HAS_ANNOTATE_JUMPTABLE sufficient?