Re: [PATCH 3/7] x86: make CX8 usage unconditional

[David Gow]

Le 06/11/2026 à 00:34, Arnd Bergmann a écrit :
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> All supported CPUs now provide the cmpxchg8b instruction, so remove
> all compile-time and runtime checks for its presence and the fallback
> implementation, to assume it just works.
>
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---

Hi Arnd,

As-is, this breaks 32-bit UML, as atomic64_cx8_32.S is removed from the 
Makefile. With that reinstated, 32-bit UML at least builds and passes 
some basic KUnit tests here (though I haven't tried it on actual 586+ 
class hardware yet, and building with CONFIG_M586TSC still fails the 
checksum tests[1]).

Reproduces with:
make O=.tmpuml ARCH=um SUBARCH=i386
or
./tools/testing/kunit/kunit.py run --arch um --kconfig_add CONFIG_64BIT=n

>   arch/um/include/asm/asm-prototypes.h  |   4 
>   arch/x86/Kconfig.cpu                  |   3 -
>   arch/x86/Kconfig.cpufeatures          |   6 +-
>   arch/x86/include/asm/asm-prototypes.h |   4 -
>   arch/x86/include/asm/atomic64_32.h    |  15 --
>   arch/x86/include/asm/cmpxchg_32.h     |  76 +---------
>   arch/x86/include/asm/percpu.h         |  14 +-
>   arch/x86/lib/Makefile                 |   4 -
>   arch/x86/lib/atomic64_386_32.S        | 195 --------------------------
>   arch/x86/lib/cmpxchg8b_emu.S          |  97 -------------
>   arch/x86/um/Kconfig                   |   2 +-
>   arch/x86/um/Makefile                  |   3 +-
>   lib/atomic64_test.c                   |   7 +-
>   13 files changed, 13 insertions(+), 417 deletions(-)
>   delete mode 100644 arch/x86/lib/atomic64_386_32.S
>   delete mode 100644 arch/x86/lib/cmpxchg8b_emu.S

...snip...

> diff --git a/arch/x86/um/Kconfig b/arch/x86/um/Kconfig
> index 44b12e45f9a0..5b78440a78b9 100644
> --- a/arch/x86/um/Kconfig
> +++ b/arch/x86/um/Kconfig
> @@ -13,7 +13,7 @@ config UML_X86
>   	select ARCH_USE_QUEUED_SPINLOCKS
>   	select DCACHE_WORD_ACCESS
>   	select HAVE_EFFICIENT_UNALIGNED_ACCESS
> -	select UML_SUBARCH_SUPPORTS_SMP if X86_CX8
> +	select UML_SUBARCH_SUPPORTS_SMP
>   
>   config 64BIT
>   	bool "64-bit kernel" if "$(SUBARCH)" = "x86"
> diff --git a/arch/x86/um/Makefile b/arch/x86/um/Makefile
> index 100b68f601f9..716da7b1e3e9 100644
> --- a/arch/x86/um/Makefile
> +++ b/arch/x86/um/Makefile
> @@ -19,8 +19,7 @@ ifeq ($(CONFIG_X86_32),y)
>   
>   obj-y += syscalls_32.o
>   
> -subarch-y = ../lib/string_32.o ../lib/atomic64_32.o ../lib/atomic64_cx8_32.o
> -subarch-y += ../lib/cmpxchg8b_emu.o ../lib/atomic64_386_32.o
> +subarch-y = ../lib/string_32.o ../lib/atomic64_32.o

Reinstate ../lib/atomic64_cx8_32.o here!

>   subarch-y += ../lib/checksum_32.o
>   subarch-y += ../kernel/sys_ia32.o
>   
> diff --git a/lib/atomic64_test.c b/lib/atomic64_test.c
> index d726068358c7..37af41df3e6a 100644
Cheers,
-- David

[1]: 
https://lore.kernel.org/all/20240720063937.2311958-2-davidgow@xxxxxxxxxx/