On Sun, Jun 25, 2023 at 3:48 PM WANG Xuerui <kernel@xxxxxxxxxx> wrote: > > On 2023/6/25 15:36, Xi Ruoyao wrote: > > On Sun, 2023-06-25 at 15:16 +0800, WANG Xuerui wrote: > >> On 2023/6/25 10:13, Huacai Chen wrote: > >>> Hi, Ruoyao, > >>> > >>> On Sun, Jun 25, 2023 at 2:42 AM WANG Xuerui <kernel@xxxxxxxxxx> wrote: > >>>> > >>>> From: WANG Xuerui <git@xxxxxxxxxx> > >>>> > >>>> Now the arch code is mostly ready for LLVM/Clang consumption, it is time > >>>> to re-organize the CFLAGS a little to actually enable the LLVM build. > >>>> > >>>> In particular, -mexplicit-relocs and -mdirect-extern-access are not > >>>> necessary nor supported on Clang; feature detection via cc-option would > >>>> not work, because that way the broken combo of "new GNU as + old GCC" > >>>> would seem to get "fixed", but actually produce broken kernels. > >>>> Explicitly depending on CONFIG_CC_IS_CLANG is thus necessary to not > >>>> regress UX for those building their own kernels. > >>>> > >>>> A build with !RELOCATABLE && !MODULE is confirmed working within a QEMU > >>>> environment; support for the two features are currently blocked on > >>>> LLVM/Clang, and will come later. > >>>> > >>>> Signed-off-by: WANG Xuerui <git@xxxxxxxxxx> > >>>> --- > >>>> arch/loongarch/Makefile | 6 +++++- > >>>> 1 file changed, 5 insertions(+), 1 deletion(-) > >>>> > >>>> diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile > >>>> index 366771016b99..82c619791a63 100644 > >>>> --- a/arch/loongarch/Makefile > >>>> +++ b/arch/loongarch/Makefile > >>>> @@ -51,7 +51,9 @@ LDFLAGS_vmlinux += -static -n -nostdlib > >>>> > >>>> # When the assembler supports explicit relocation hint, we must use it. > >>>> # GCC may have -mexplicit-relocs off by default if it was built with an old > >>>> -# assembler, so we force it via an option. > >>>> +# assembler, so we force it via an option. For LLVM/Clang the desired behavior > >>>> +# is the default, and the flag is not supported, so don't pass it if Clang is > >>>> +# being used. > >>>> # > >>>> # When the assembler does not supports explicit relocation hint, we can't use > >>>> # it. Disable it if the compiler supports it. > >>>> @@ -61,8 +63,10 @@ LDFLAGS_vmlinux += -static -n -nostdlib > >>>> # combination of a "new" assembler and "old" compiler is not supported. Either > >>>> # upgrade the compiler or downgrade the assembler. > >>>> ifdef CONFIG_AS_HAS_EXPLICIT_RELOCS > >>>> +ifndef CONFIG_CC_IS_CLANG > >>>> cflags-y += -mexplicit-relocs > >>>> KBUILD_CFLAGS_KERNEL += -mdirect-extern-access > >>>> +endif > >>> I prefer to drop CONFIG_CC_IS_CLANG and use > >>> cflags-y += $(call cc-option,-mexplicit-relocs) > >>> KBUILD_CFLAGS_KERNEL += $(call cc-option,-mdirect-extern-access) > >>> > >>> Then Patch-6 can be merged in this. > >>> > >>> What's your opinion? > >> > >> FYI: with this approach the build no longer instantly dies with binutils > >> 2.40 + gcc 12.3, but there are also tons of warnings that say the model > >> attribute is being ignored. I checked earlier discussions and this means > >> modules are silently broken at runtime, which is not particularly good UX. > > > > We can add > > > > #if defined(MODULE) && !__has_attribute(model) > > # error some fancy error message > > #endif > > > > into percpu.h to error out in this case. It had been in my earlier > > drafts of explicit relocs patches, but we dropped it because there was > > no such configuration (unless a snapshot of development GCC is used, and > > using such a snapshot is never supported IIUC). > > Ah I've seen that. So in this case we simply wrap -mexplicit-relocs with > cc-option and error out in case of CONFIG_MODULE but no model attribute, > which nicely prevents broken configurations (MODULE && ((old_gcc && > new_binutils) || clang)) with feature detection alone. > > This seems elegant and better to me; Huacai, WDYT? OK, perfect. Huacai > > -- > WANG "xen0n" Xuerui > > Linux/LoongArch mailing list: https://lore.kernel.org/loongarch/ >
