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

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

On 7/9/2026 5:36 AM, Miguel Ojeda wrote:
> On Wed, Jul 8, 2026 at 11:40 PM Chang S. Bae <chang.seok.bae@intel.com> wrote:
>>
>> +        KBUILD_RUSTFLAGS += -Ctarget-cpu=native $(if $(call rust-min-version,109100),-Ctarget-feature=-apxf,)
> 
> Hmm... I don't think this was tested?
> 
> There is a missing `c` there -- the flag is never going to get passed.

Ouch. :(

> 
> And while it is true that `rustc` knows about the target feature since
> 1.88.0, it will (sadly) still loudly warn about it:
> 
>      warning: unstable feature specified for `-Ctarget-feature`: `apxf`
>        |
>        = note: this feature is not stably supported; its behavior can
> change in the future
> 
> Instead, we should be able to do it in the custom target spec, i.e. in
> `scripts/generate_rust_target.rs`, assuming `-Ctarget-cpu=native`
> enables it and we need to override it. But please double-check the
> interaction between those and test that LLVM is actually getting the
> right set of features you want.

I did some investigation [*] into the APX code generation across Rust 
versions. A few notable observations:

  * Rust 1.88 is the first release to recognizes the apxf target feature.
    Prior to that, when LLVM supports APX, target-cpu=native appears to
    allow APX code generation.
  * Starting with Rust 1.95, target-cpu=native no longer appears to
    enable APX instruction generation even without explicitly disabling
    apxf. However, this seems to be implementation-specific and could
    change in future releases.
  * Passing target-feature=-apxf currently emits the warning you quoted
    because the feature is still unstable. Using the generated target
    JSON avoids the warning but Rust versions prior to 1.93 instead
    produce another noise:

    '-apxf' is not a recognized feature for this target ...

Given that, the goal here is to disable APX without build warnings. Rust
1.93 needs to be the minimum version via the generated target JSON.

> 
> Finally, we are trying to get rid of the custom target and instead use
> flags as soon as possible, so if the flag will be eventually needed,
> then it should be stabilized.
> 
> To help with that, I have tagged the tracking issue with our Rust for
> Linux tag and will raise it to them in our next meeting, but it is
> even better if the actual company pings as well:
> 
>    https://github.com/rust-lang/rust/issues/139284
> 
> I have also added it to our usual live list of features:
> 
>    https://github.com/Rust-for-Linux/linux/issues/2
> 
> Link: https://github.com/rust-lang/rust/issues/139284
> Link: https://github.com/rust-lang/rust/pull/139534
> 
> Also Cc'ing rust-for-linux and the maintainers and reviewers.
> 
> I hope this helps.
Indeed. This is very helpful!

Thanks,
Chang

[*]:
https://github.com/intel/apx/blob/study_rust-apxf/study_rust-apxf.md