arm64: Use static keys for CPU features

This patch adds static keys transparently for all the cpu_hwcaps
features by implementing an array of default-false static keys and
enabling them when detected. The cpus_have_cap() check uses the static
keys if the feature being checked is a constant, otherwise the compiler
generates the bitmap test.

Because of the early call to static_branch_enable() via
check_local_cpu_errata() -> update_cpu_capabilities(), the jump labels
are initialised in cpuinfo_store_boot_cpu().

Cc: Will Deacon <will.deacon@arm.com>
Cc: Suzuki K. Poulose <Suzuki.Poulose@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/cpufeature.h

@@ -9,6 +9,8 @@
 #ifndef __ASM_CPUFEATURE_H
 #define __ASM_CPUFEATURE_H
 
+#include <linux/jump_label.h>
+
 #include <asm/hwcap.h>
 #include <asm/sysreg.h>
 
@@ -110,6 +112,7 @@ struct arm64_cpu_capabilities {
 };
 
 extern DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS);
+extern struct static_key_false cpu_hwcap_keys[ARM64_NCAPS];
 
 bool this_cpu_has_cap(unsigned int cap);
 
@@ -122,16 +125,21 @@ static inline bool cpus_have_cap(unsigned int num)
 {
 	if (num >= ARM64_NCAPS)
 		return false;
+	if (__builtin_constant_p(num))
+		return static_branch_unlikely(&cpu_hwcap_keys[num]);
+	else
 		return test_bit(num, cpu_hwcaps);
 }
 
 static inline void cpus_set_cap(unsigned int num)
 {
-	if (num >= ARM64_NCAPS)
+	if (num >= ARM64_NCAPS) {
 		pr_warn("Attempt to set an illegal CPU capability (%d >= %d)\n",
 			num, ARM64_NCAPS);
-	else
+	} else {
 		__set_bit(num, cpu_hwcaps);
+		static_branch_enable(&cpu_hwcap_keys[num]);
+	}
 }
 
 static inline int __attribute_const__

arch/arm64/kernel/cpufeature.c

@@ -46,6 +46,9 @@ unsigned int compat_elf_hwcap2 __read_mostly;
 
 DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS);
 
+DEFINE_STATIC_KEY_ARRAY_FALSE(cpu_hwcap_keys, ARM64_NCAPS);
+EXPORT_SYMBOL(cpu_hwcap_keys);
+
 #define __ARM64_FTR_BITS(SIGNED, STRICT, TYPE, SHIFT, WIDTH, SAFE_VAL) \
 	{						\
 		.sign = SIGNED,				\

arch/arm64/kernel/smp.c

@@ -437,6 +437,11 @@ void __init smp_cpus_done(unsigned int max_cpus)
 void __init smp_prepare_boot_cpu(void)
 {
 	set_my_cpu_offset(per_cpu_offset(smp_processor_id()));
+	/*
+	 * Initialise the static keys early as they may be enabled by the
+	 * cpufeature code.
+	 */
+	jump_label_init();
 	cpuinfo_store_boot_cpu();
 	save_boot_cpu_run_el();
 }