Re: [PATCH v4 1/3] arm64: cpufeature: introduce helper cpu_has_hw_af()

[Matthew Wilcox]

On Wed, Sep 18, 2019 at 09:19:12PM +0800, Jia He wrote:
> +/* Decouple AF from AFDBM. */
> +bool cpu_has_hw_af(void)
> +{
> +	return (read_cpuid(ID_AA64MMFR1_EL1) & 0xf);
> +}
> +

Do you really want to call read_cpuid() every time?  I would have thought
you'd want to use the static branch mechanism to do the right thing at
boot time.  See Documentation/static-keys.txt.