How to disable NEON in kernel5.4.55 for ARM64

Ard Biesheuvel ardb at kernel.org
Fri Nov 20 06:38:45 EST 2020


On Fri, 20 Nov 2020 at 11:17, Suzuki K Poulose <suzuki.poulose at arm.com> wrote:
>
> Hi Trey,
>
> On 11/17/20 7:13 AM, Trey Chen (陳振華) wrote:
> > Dear linux-arm-kernel,
> >
> > We use CA-53 CPU and want to run on ARM64 Kernel5.4.55.
> > For some reason, we remove NEON from CA-53.
> >
> > Do you know how to disable NEON in kernel5.4.55 for ARM64?
>
> Ideally you don't need any changes to the kernel (with all the fixes
> in [0]) to run on a "no-fpsimd" system. The kernel detects the missing
> fpsimd at runtime and takes the necessary steps to skip accessing the
> NEON registers.
>
> The userspace must rely on the HWCAP bits to detect whether the FP/SIMD
> is supported (This is not true in the normal GNU libc, as no-fpsimd
> is not part of the ABI). So, as long as the userspace doesn't access
> fpsimd blindly you shouldn't need any changes.
>
>

This basically means that you can only run a softfloat 32-bit ARM user
space, or create your own from [almost] scratch.



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