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

[Tiezhu Yang]

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@xxxxxxxxxxx> 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.

(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