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

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

[Uros Bizjak]

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@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Justin Stitt <justinstitt@google.com>
---
 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));			\
+})
 
 #else /* !CONFIG_USE_X86_SEG_SUPPORT: */
 
-- 
2.42.0

[PATCH 2/2] x86/percpu: *DO NOT MERGE* Enable named address spaces for clang

[Uros Bizjak]

Enable named address spaces feature for clang.

However, someone from clang side should qualify the compiler
before this feature is enabled for real.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Justin Stitt <justinstitt@google.com>
---
 arch/x86/Kconfig | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index e8837116704c..08455e62f994 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2433,7 +2433,6 @@ endmenu
 
 config CC_HAS_NAMED_AS
 	def_bool $(success,echo 'int __seg_fs fs; int __seg_gs gs;' | $(CC) -x c - -S -o /dev/null)
-	depends on CC_IS_GCC
 
 config CC_HAS_NAMED_AS_FIXED_SANITIZERS
 	def_bool CC_IS_GCC && GCC_VERSION >= 130300
-- 
2.42.0

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

[Nathan Chancellor]

On Sun, May 26, 2024 at 07:55:52PM +0200, Uros Bizjak 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@gmail.com>

Thanks, this resolves the warning for me when patch 2 is applied.

Acked-by: Nathan Chancellor <nathan@kernel.org>

However, as both of us have noticed [1][2], there is a problem in LLVM's
x86 backend with the address space casting that the kernel does, so
patch 2 should not be merged.

[1]: https://github.com/ClangBuiltLinux/linux/issues/2013
[2]: https://github.com/llvm/llvm-project/issues/93449

> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Bill Wendling <morbo@google.com>
> Cc: Justin Stitt <justinstitt@google.com>
> ---
>  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));			\
> +})
>  
>  #else /* !CONFIG_USE_X86_SEG_SUPPORT: */
>  
> -- 
> 2.42.0
> 

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

[Ingo Molnar]

* Uros Bizjak <ubizjak@gmail.com> 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@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Bill Wendling <morbo@google.com>
> Cc: Justin Stitt <justinstitt@google.com>
> ---
>  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?

Thanks,

	Ingo

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@kernel.org> wrote:
>
>
> * Uros Bizjak <ubizjak@gmail.com> 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@gmail.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > Cc: Borislav Petkov <bp@alien8.de>
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Cc: Nathan Chancellor <nathan@kernel.org>
> > Cc: Nick Desaulniers <ndesaulniers@google.com>
> > Cc: Bill Wendling <morbo@google.com>
> > Cc: Justin Stitt <justinstitt@google.com>
> > ---
> >  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.