Re: [PATCH] x86/build/64: Prevent native builds from generating SSE/AVX code

[Antoni Kuczyński]

Hi Nathan,

On Sun, Oct 25, 2026 at 12:44:11PM -0700, Nathan Chancellor wrote:
> This option has been in the kernel for quite some time and I am sure
> people are using it. I wonder why this has not come up sooner. Just
> unlucky timing or configuration?

Configuration, on my side, sorry for the noise. Following up on your
question I found that "gcc" in my PATH was the icecc (icecream 1.4.0)
wrapper. To be able to distribute the compilation, icecc replaces
-march=native with an explicit list of options for the local CPU:

  /usr/bin/gcc -mno-sse -mno-avx -Os -march=ivybridge -mmmx -mpopcnt
    -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -mavx ...

so the expanded -msse/-mavx come after the kernel's -mno-* flags and
win. Plain GCC 14.2 does not have this problem: the reproducer from
the changelog built with /usr/bin/gcc-14 emits no SIMD instructions,
as explicit -mno-* options take precedence over what -march=native
enables.

So the changelog's explanation and the Fixes:/stable tags are wrong,
please drop this patch. I will report the issue to icecream instead.

> Maintaining two copies of these flags (and having them appear in the
> command line twice) seems objectively worse than just adding an extra
> 'ifdef CONFIG_X86_64' and moving the block up so that this happens in
> the naturally expected order?

Agreed, and FWIW your diff on top of current master also fixes the
icecc build here: -march=native now comes before the -mno-* flags and
arch/x86/boot/startup/gdt_idt.o contains no SIMD instructions. If you
think that ordering is worth having on its own merits, I am happy to
send it as a cleanup without Fixes:/stable, but I understand if it is
not worth the churn for an icecc-only problem.

Thanks for the review,
Antoni