How to disable NEON in kernel5.4.55 for ARM64

[Suzuki K Poulose]

On 11/20/20 10:16 AM, Suzuki K Poulose 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.
> 

[0] https://lore.kernel.org/linux-arm-kernel/20200114115613.GC2289@mbp/T/

Suzuki