Re: [PATCH] x86/build/64: Prevent native builds from generating APX instructions

[Nathan Chancellor]

On Wed, Jul 08, 2026 at 09:14:35PM +0000, Chang S. Bae wrote:
> Omar reported this broad concern to me, when resolving a separate issue
> with his custom module. CONFIG_X86_NATIVE_CPU=y allows builds to
> opportunistically emit APX instructions when the build host supports APX
> since commit:
> 
>   ea1dcca1de12 ("x86/kbuild/64: Add the CONFIG_X86_NATIVE_CPU option to locally optimize the kernel with '-march=native'")
> 
> The kernel is not yet prepared to use APX internally. For example, there
> is no context-switch support for general in-kernel use of the extended
> GPRs.
> 
> Explicitly disable APX when building with `-march=native`.
> 
> Since GCC 14 and LLVM 18, both compilers support APX. LLVM 19 is already
> the minimum version to support native builds from:
> 
>   ad9b861824ac ("x86/kbuild/64: Restrict clang versions that can use '-march=native'")
> 
> RUST supports APX detection via XCR0 since v1.91 release. While the
> support is not official yet, conservatively set that version as the
> minimum.
> 
> Reported-by: Omar Avelar <omar.avelar@xxxxxxxxx>
> Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # v6.16+
> Cc: Miguel Ojeda <ojeda@xxxxxxxxxx>
> Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
> ---

Miguel's comment aside:

Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>

Some style feedback below.

> ---
>  arch/x86/Makefile | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 598f178102ee..256948e65073 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -161,7 +161,15 @@ else
>  
>  ifdef CONFIG_X86_NATIVE_CPU
>          KBUILD_CFLAGS += -march=native
> -        KBUILD_RUSTFLAGS += -Ctarget-cpu=native
> +        # Do not generate APX instructions as in-kernel use isn't ready
> +  ifdef CONFIG_CC_IS_GCC
> +        KBUILD_CFLAGS += $(if $(call gcc-min-version,140000),-mno-apxf,)
> +  endif
> +  ifdef CONFIG_CC_IS_CLANG
> +        # The minimum version for native build already supports the option
> +        KBUILD_CFLAGS += -mno-apxf
> +  endif

As the flag is the same between the two compilers, I think it would be
more readable to do something like:

  # Do not generate APX instructions as in-kernel use isn't ready
  # Supported by GCC 14+ and LLVM 18+
  KBUILD_CFLAGS += $(call cc-option,-mno-apxf)

I realize this was probably written to intentionally avoid calling
cc-option but I am not sure it is worth micro-optimizing the build like
this. Another option that avoids the separate blocks and cc-option would
be something like:

  KBUILD_CFLAGS += $(if $(call gcc-min-version,140000)$(CONFIG_CC_IS_CLANG),-mno-apxf)

That said, it is ultimately up to the -tip folks.

> +        KBUILD_RUSTFLAGS += -Ctarget-cpu=native $(if $(call rust-min-version,109100),-Ctarget-feature=-apxf,)

The trailing comma is not necessary for this if, it is implicit.

>  else
>          KBUILD_CFLAGS += -march=x86-64 -mtune=generic
>          KBUILD_RUSTFLAGS += -Ctarget-cpu=x86-64 -Ztune-cpu=generic
> -- 
> 2.51.0
> 

-- 
Cheers,
Nathan