Messages in this thread | | | Date | Wed, 18 Sep 2019 17:49:25 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH v4 1/3] arm64: cpufeature: introduce helper cpu_has_hw_af() |
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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
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