linux-kernel - Re: [PATCH v2 3/4] objtool/LoongArch: Fix unreachable instruction warnings about entry points

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Message-ID: <CAAhV-H738P94-XJQx=KMH5oGfp_eji_edvKr1HAgK3mnVjh3Mw@mail.gmail.com>
Date: Wed, 17 Sep 2025 18:16:09 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, Peter Zijlstra <peterz@...radead.org>, loongarch@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/4] objtool/LoongArch: Fix unreachable instruction
 warnings about entry points

On Wed, Sep 17, 2025 at 5:47 PM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>
> On 2025/9/17 下午3:07, Huacai Chen wrote:
> > Hi, Tiezhu,
> >
> > On Wed, Sep 17, 2025 at 9:10 AM Tiezhu Yang <yangtiezhu@...ngson.cn> 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 fix 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.
>
> ...
>
> >> -OBJECT_FILES_NON_STANDARD_head.o := y
> > This line should be kept, othewise we get:
> >
> > arch/loongarch/kernel/head.o: warning: objtool: kernel_entry+0xf4:
> > start_kernel() missing __noreturn in .c/.h or NORETURN() in
> > noreturns.h
> >
> > even without LTO. This is a regression, we can only remove it after
> > the above warning be fixed.
>
> As said in the commit message, ASM_BUG() needs to be removed
> to fix the above warning.
>
> I tested again with GCC and LLVM (with and without LTO),
> there is no the warning what you said, please double check.
I'm sorry, I was confused by the similar warnings described in this
patch and the cover letter.

Huacai

>
> (1) GCC
> make ARCH=loongarch defconfig
> make ARCH=loongarch -j8
>
> (2) LLVM without LTO
> make ARCH=loongarch LLVM=1 clean defconfig
> make ARCH=loongarch LLVM=1 olddefconfig all -j8
>
> (3) LLVM with LTO
> make ARCH=loongarch LLVM=1 clean defconfig
> scripts/config -d LTO_NONE -e LTO_CLANG_THIN
> make ARCH=loongarch LLVM=1 olddefconfig all -j8
>
> Thanks,
> Tiezhu
>