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

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Message-ID: <ZpfX4KelYaXsHlHe@gmail.com>
Date: Wed, 17 Jul 2024 16:40:32 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Uros Bizjak <ubizjak@...il.com>
Cc: x86@...nel.org, llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>
Subject: Re: [PATCH 1/2] x86/percpu: Fix "multiple identical address spaces
 specified for type" clang warning


* Uros Bizjak <ubizjak@...il.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@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: Bill Wendling <morbo@...gle.com>
> Cc: Justin Stitt <justinstitt@...gle.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