[PATCH v2 2/2] arm64: Use static keys for CPU features

[Catalin Marinas]

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.dea...@arm.com>
Cc: Suzuki K. Poulose <suzuki.poul...@arm.com>
Signed-off-by: Catalin Marinas <catalin.mari...@arm.com>
---
 arch/arm64/include/asm/cpufeature.h | 14 +++++++++++---
 arch/arm64/kernel/cpufeature.c      |  3 +++
 arch/arm64/kernel/smp.c             |  5 +++++
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h 
b/arch/arm64/include/asm/cpufeature.h
index 7099f26e3702..c9dfb1e4c435 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/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>
 
@@ -109,6 +111,7 @@ 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);
 
@@ -121,16 +124,21 @@ static inline bool cpus_have_cap(unsigned int num)
 {
        if (num >= ARM64_NCAPS)
                return false;
-       return test_bit(num, cpu_hwcaps);
+       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__
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 62272eac1352..919b2d0d68ae 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/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,                         \
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index d93d43352504..c3c08368a685 100644
--- a/arch/arm64/kernel/smp.c
+++ b/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();
 }