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

[Chang S. Bae]

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@xxxxxxxxx> 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@xxxxxxxxxxxxxxx

Yes, I have included it.

Thanks,
Chang