Re: [PATCH v4 1/3] arm64: cpufeature: introduce helper cpu_has_hw_af()
From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2019-09-18 16:49:32
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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:quoted
+/* 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel