linux-kernel - Re: [PATCH v1 3/3] LoongArch: Fix unreachable instruction warnings about entry functions

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Message-ID: <20250901082017.GC4067720@noisy.programming.kicks-ass.net>
Date: Mon, 1 Sep 2025 10:20:17 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Huacai Chen <chenhuacai@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>, loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 3/3] LoongArch: Fix unreachable instruction warnings
 about entry functions

On Mon, Sep 01, 2025 at 03:21:56PM +0800, Tiezhu Yang wrote:
> When compiling with LLVM and CONFIG_LTO_CLANG is set, there exist the
> following objtool warnings:
> 
>   vmlinux.o: warning: objtool: kernel_entry+0x0: unreachable instruction
>   vmlinux.o: warning: objtool: smpboot_entry+0x0: unreachable instruction
> 
> kernel_entry() and smpboot_entry() are in arch/loongarch/kernel/head.S,
> there is "OBJECT_FILES_NON_STANDARD_head.o := y" to skip objtool checking
> for head.o, but the STACK_FRAME_NON_STANDARD macro does not work for link
> time validation of vmlinux.o according to objtool documentation, just give
> a proper unwind hint to silence the warnings.
> 
> By the way, ASM_BUG() can be removed due to unnecessary, otherwise there
> are following warnings:
> 
>   kernel_entry+0xf4: start_kernel() missing __noreturn
>   in .c/.h or NORETURN() in noreturns.h
> 
>   smpboot_entry+0x68: start_secondary() missing __noreturn
>   in .c/.h or NORETURN() in noreturns.h
> 
> This is because the previous instructions of kernel_entry+0xf4 and
> smpboot_entry+0x68 are the 'bl' instructions, start_kernel() and
> start_secondary() are the respective call destination symbols which
> are noreturn functions, then the 'bl' instructions are already marked
> as dead end in annotate_call_site().
> 
> For now, it is time to remove "OBJECT_FILES_NON_STANDARD_head.o := y"
> in arch/loongarch/kernel/Makefile.
> 
> Link: https://lore.kernel.org/lkml/20250814083651.GR4067720@noisy.programming.kicks-ass.net/
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>

Right, this looks good.

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

> ---
>  arch/loongarch/kernel/Makefile | 2 --
>  arch/loongarch/kernel/head.S   | 6 ++----
>  2 files changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/loongarch/kernel/Makefile b/arch/loongarch/kernel/Makefile
> index 6f5a4574a911..4302c5b0a201 100644
> --- a/arch/loongarch/kernel/Makefile
> +++ b/arch/loongarch/kernel/Makefile
> @@ -3,8 +3,6 @@
>  # Makefile for the Linux/LoongArch kernel.
>  #
>  
> -OBJECT_FILES_NON_STANDARD_head.o := y
> -
>  always-$(KBUILD_BUILTIN)	:= vmlinux.lds
>  
>  obj-y		+= head.o cpu-probe.o cacheinfo.o env.o setup.o entry.o genex.o \
> diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
> index e3865e92a917..a11880f3a7e1 100644
> --- a/arch/loongarch/kernel/head.S
> +++ b/arch/loongarch/kernel/head.S
> @@ -42,6 +42,7 @@ SYM_DATA(kernel_fsize, .long _kernel_fsize);
>  	.align 12
>  
>  SYM_CODE_START(kernel_entry)			# kernel entry point
> +	UNWIND_HINT_UNDEFINED
>  
>  	/* Config direct window and set PG */
>  	SETUP_DMWINS	t0
> @@ -109,8 +110,6 @@ SYM_CODE_START(kernel_entry)			# kernel entry point
>  #endif
>  
>  	bl		start_kernel
> -	ASM_BUG()
> -
>  SYM_CODE_END(kernel_entry)
>  
>  #ifdef CONFIG_SMP
> @@ -120,6 +119,7 @@ SYM_CODE_END(kernel_entry)
>   * function after setting up the stack and tp registers.
>   */
>  SYM_CODE_START(smpboot_entry)
> +	UNWIND_HINT_UNDEFINED
>  
>  	SETUP_DMWINS	t0
>  	JUMP_VIRT_ADDR	t0, t1
> @@ -142,8 +142,6 @@ SYM_CODE_START(smpboot_entry)
>  	ld.d		tp, t0, CPU_BOOT_TINFO
>  
>  	bl		start_secondary
> -	ASM_BUG()
> -
>  SYM_CODE_END(smpboot_entry)
>  
>  #endif /* CONFIG_SMP */
> -- 
> 2.42.0
>