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

From: Jason Baron
Date: Wed Sep 07 2016 - 13:00:32 EST


On 09/05/2016 01:25 PM, Catalin Marinas wrote:
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().

Was there a reason the jump_label_init() couldn't be moved
earlier in the common code?

Thanks,

-Jason


Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Suzuki K. Poulose <Suzuki.Poulose@xxxxxxx>
Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
---
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();
}