2024-05-26 17:57:16

by Uros Bizjak

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

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 <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Bill Wendling <[email protected]>
Cc: Justin Stitt <[email protected]>
---
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

2024-05-26 17:57:19

by Uros Bizjak

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

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 <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Bill Wendling <[email protected]>
Cc: Justin Stitt <[email protected]>
---
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

2024-05-30 15:58:08

by Nathan Chancellor

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

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 <[email protected]>

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

Acked-by: Nathan Chancellor <[email protected]>

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 <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Borislav Petkov <[email protected]>
> Cc: Dave Hansen <[email protected]>
> Cc: "H. Peter Anvin" <[email protected]>
> Cc: Nathan Chancellor <[email protected]>
> Cc: Nick Desaulniers <[email protected]>
> Cc: Bill Wendling <[email protected]>
> Cc: Justin Stitt <[email protected]>
> ---
> 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
>