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.
Link: https://lore.kernel.org/lkml/20250814083651.GR4067720@noisy.programming.kicks-ass.net/
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
---
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 6ce999586757..c62dab32a06b 100644
--- a/arch/loongarch/kernel/head.S
+++ b/arch/loongarch/kernel/head.S
@@ -43,6 +43,7 @@ SYM_DATA(kernel_fsize, .long _kernel_fsize);
.align 12
SYM_CODE_START(kernel_entry) # kernel entry point
+ UNWIND_HINT_END_OF_STACK
/* Config direct window and set PG */
SETUP_DMWINS t0
@@ -110,8 +111,6 @@ SYM_CODE_START(kernel_entry) # kernel entry point
#endif
bl start_kernel
- ASM_BUG()
-
SYM_CODE_END(kernel_entry)
#ifdef CONFIG_SMP
@@ -121,6 +120,7 @@ SYM_CODE_END(kernel_entry)
* function after setting up the stack and tp registers.
*/
SYM_CODE_START(smpboot_entry)
+ UNWIND_HINT_END_OF_STACK
SETUP_DMWINS t0
JUMP_VIRT_ADDR t0, t1
@@ -143,8 +143,6 @@ 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 */
--
2.42.0Hi, Tiezhu, On Wed, Sep 17, 2025 at 9:10 AM Tiezhu Yang <yangtiezhu@loongson.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. > > Link: https://lore.kernel.org/lkml/20250814083651.GR4067720@noisy.programming.kicks-ass.net/ > Suggested-by: Peter Zijlstra <peterz@infradead.org> > Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn> > --- > 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 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. Huacai > - > 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 6ce999586757..c62dab32a06b 100644 > --- a/arch/loongarch/kernel/head.S > +++ b/arch/loongarch/kernel/head.S > @@ -43,6 +43,7 @@ SYM_DATA(kernel_fsize, .long _kernel_fsize); > .align 12 > > SYM_CODE_START(kernel_entry) # kernel entry point > + UNWIND_HINT_END_OF_STACK > > /* Config direct window and set PG */ > SETUP_DMWINS t0 > @@ -110,8 +111,6 @@ SYM_CODE_START(kernel_entry) # kernel entry point > #endif > > bl start_kernel > - ASM_BUG() > - > SYM_CODE_END(kernel_entry) > > #ifdef CONFIG_SMP > @@ -121,6 +120,7 @@ SYM_CODE_END(kernel_entry) > * function after setting up the stack and tp registers. > */ > SYM_CODE_START(smpboot_entry) > + UNWIND_HINT_END_OF_STACK > > SETUP_DMWINS t0 > JUMP_VIRT_ADDR t0, t1 > @@ -143,8 +143,6 @@ 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 >
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@loongson.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. (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
On Wed, Sep 17, 2025 at 5:47 PM Tiezhu Yang <yangtiezhu@loongson.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@loongson.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 >
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