Re: [PATCH 1/2] x86/percpu: Fix "multiple identical address spaces specified for type" clang warning

[Uros Bizjak]

On Wed, Jul 17, 2024 at 4:40 PM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>
> * Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
>
> > The clang build with named address spaces enabled currently fails with:
> >
> > error: multiple identical address spaces specified for type [-Werror,-Wduplicate-decl-specifier]
> >
> > The warning is emitted when accessing const_pcpu_hot structure,
> > which is already declared in __seg_gs named address space.
> >
> > Use specialized accessor for __raw_cpu_read_const() instead, avoiding
> > redeclaring __seg_gs named address space via __raw_cpu_read().
> >
> > Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
> > Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> > Cc: Bill Wendling <morbo@xxxxxxxxxx>
> > Cc: Justin Stitt <justinstitt@xxxxxxxxxx>
> > ---
> >  arch/x86/include/asm/percpu.h | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
> > index c55a79d5feae..aeea5c8a17de 100644
> > --- a/arch/x86/include/asm/percpu.h
> > +++ b/arch/x86/include/asm/percpu.h
> > @@ -160,7 +160,10 @@ do {                                                                     \
> >       *(qual __my_cpu_type(pcp) *)__my_cpu_ptr(&(pcp)) = (val);       \
> >  } while (0)
> >
> > -#define __raw_cpu_read_const(pcp)    __raw_cpu_read(, , pcp)
> > +#define __raw_cpu_read_const(pcp)                                    \
> > +({                                                                   \
> > +     *(typeof(pcp) *)(__force uintptr_t)(&(pcp));                    \
> > +})
>
> So shouldn't this be carried together with the patch that enables named
> address spaces on Clang? Is it possible that this patch might not be needed
> in a Clang version where named address spaces work as expected?

No, as diagnosed by Clang, there are indeed two identical specifiers,
so the patch also applies as a general patch.

Regarding Clang, before enabling the feature, issue #93449 [1]
(internal compiler error) has to be fixed.

[1] https://github.com/llvm/llvm-project/issues/93449

Uros.