Re: [PATCH v2] x86/build/64: Prevent native builds from generating APX instructions

["Chang S. Bae" <chang.seok.bae@intel.com>]

On 8/7/2026 9:58 AM, Miguel Ojeda wrote:
> On Fri, Aug 7, 2026 at 5:43 PM Chang S. Bae <chang.seok.bae@intel.com> 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 the commit.
> 
> I think this sentence may be cut? I guess this refers to the commit in
> the Fixes tag.


Yes, I intended “the commit” to refer to the one in the Fixes: tag. To 
make it explicit, I can add the commit back here:

   ea1dcca1de12 ("x86/kbuild/64: Add the CONFIG_X86_NATIVE_CPU ...")


>> For Rust, APX must be explicitly disabled because any backend supporting
>> APX may otherwise emit APX instructions with target-cpu=native. Pass
>> features=-apxf through the generated JSON to avoid unstable-feature
>> warnings.
> 
> So, just to confirm: is Rust 1.88 (with a new enough LLVM) enough to
> avoid the warnings? Maybe a link to the tracking issue would be nice:
> 
>    https://github.com/rust-lang/rust/issues/139284


Passing `-apxf` through the generated target JSON avoids this warning:

warning: unstable feature specified for `-Ctarget-feature`: `apxf`
   |
   = note: this feature is not stably supported; its behavior can change 
in the future

I’ll add the link to reference the status of the Rust support. Thanks.

>> Support for this gating also depends on the Rust/LLVM combination. Rust
>> 1.88 introduced the `apxf` feature option, but versions prior to 1.93 may
>> emit an `apxf` attribute which LLVM 23 or later can interpret. Restrict
>> native Rust builds accordingly.
>>
>> Fixes: ea1dcca1de12 ("x86/kbuild/64: Add the CONFIG_X86_NATIVE_CPU option to locally optimize the kernel with '-march=native'")
> 
> This looks like it exists in 6.18.y+, so I assume this should have:
> 
> Cc: stable@vger.kernel.org

Yes, I have included it.

Thanks,
Chang