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

[Catalin Marinas]

On Wed, Sep 18, 2019 at 07:20:17AM -0700, Matthew Wilcox wrote:
> 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.

We do have a static branch mechanism for other CPU features but mostly
because on some big.LITTLE systems, the CPUs may differ slightly so we
only advertise the common features.

I guess here the additional instructions for reading and checking the
CPUID here would be lost in the noise compared to the actual page
copying.

-- 
Catalin