x86: A fast way to check capabilities of the current cpu

From: Christoph Lameter
Date: Wed Dec 15 2010 - 15:07:52 EST


Subject: x86: A fast way to check capabilities of the current cpu

Add this_cpu_has() which determines if the current cpu has a certain
ability using a segment prefix and a bit test operation.

For that we need to add bit operations to x86s percpu.h.

Many uses of cpu_has use a pointer passed to a function to determine
the current flags. That is no longer necessary after this patch.

However, this patch only converts the straightforward cases where
cpu_has is used with this_cpu_ptr. The rest is work for later.

Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>

---
arch/x86/include/asm/cpufeature.h | 13 +++++++++----
arch/x86/include/asm/percpu.h | 27 +++++++++++++++++++++++++++
arch/x86/kernel/apic/apic.c | 2 +-
arch/x86/kernel/process.c | 4 ++--
arch/x86/kernel/smpboot.c | 4 ++--
5 files changed, 41 insertions(+), 9 deletions(-)

Index: linux-2.6/arch/x86/include/asm/cpufeature.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/cpufeature.h 2010-12-15 12:38:52.000000000 -0600
+++ linux-2.6/arch/x86/include/asm/cpufeature.h 2010-12-15 12:54:48.000000000 -0600
@@ -206,8 +206,7 @@ extern const char * const x86_power_flag
#define test_cpu_cap(c, bit) \
test_bit(bit, (unsigned long *)((c)->x86_capability))

-#define cpu_has(c, bit) \
- (__builtin_constant_p(bit) && \
+#define REQUIRED_MASK_BIT_SET(bit) \
( (((bit)>>5)==0 && (1UL<<((bit)&31) & REQUIRED_MASK0)) || \
(((bit)>>5)==1 && (1UL<<((bit)&31) & REQUIRED_MASK1)) || \
(((bit)>>5)==2 && (1UL<<((bit)&31) & REQUIRED_MASK2)) || \
@@ -217,10 +216,16 @@ extern const char * const x86_power_flag
(((bit)>>5)==6 && (1UL<<((bit)&31) & REQUIRED_MASK6)) || \
(((bit)>>5)==7 && (1UL<<((bit)&31) & REQUIRED_MASK7)) || \
(((bit)>>5)==8 && (1UL<<((bit)&31) & REQUIRED_MASK8)) || \
- (((bit)>>5)==9 && (1UL<<((bit)&31) & REQUIRED_MASK9)) ) \
- ? 1 : \
+ (((bit)>>5)==9 && (1UL<<((bit)&31) & REQUIRED_MASK9)) )
+
+#define cpu_has(c, bit) \
+ (__builtin_constant_p(bit) && REQUIRED_MASK_BIT_SET(bit) ? 1 : \
test_cpu_cap(c, bit))

+#define this_cpu_has(bit) \
+ (__builtin_constant_p(bit) && REQUIRED_MASK_BIT_SET(bit) ? 1 : \
+ this_cpu_test_bit(bit, (unsigned long *)&cpu_info.x86_capability))
+
#define boot_cpu_has(bit) cpu_has(&boot_cpu_data, bit)

#define set_cpu_cap(c, bit) set_bit(bit, (unsigned long *)((c)->x86_capability))
Index: linux-2.6/arch/x86/kernel/apic/apic.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/apic/apic.c 2010-12-15 12:38:52.000000000 -0600
+++ linux-2.6/arch/x86/kernel/apic/apic.c 2010-12-15 12:38:53.000000000 -0600
@@ -516,7 +516,7 @@ static void __cpuinit setup_APIC_timer(v
{
struct clock_event_device *levt = &__get_cpu_var(lapic_events);

- if (cpu_has(__this_cpu_ptr(&cpu_info), X86_FEATURE_ARAT)) {
+ if (this_cpu_has(X86_FEATURE_ARAT)) {
lapic_clockevent.features &= ~CLOCK_EVT_FEAT_C3STOP;
/* Make LAPIC timer preferrable over percpu HPET */
lapic_clockevent.rating = 150;
Index: linux-2.6/arch/x86/include/asm/percpu.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/percpu.h 2010-12-15 12:38:52.000000000 -0600
+++ linux-2.6/arch/x86/include/asm/percpu.h 2010-12-15 13:06:27.000000000 -0600
@@ -545,6 +545,33 @@ do { \
old__; \
})

+static __always_inline int this_cpu_constant_test_bit(unsigned int nr,
+ const unsigned long __percpu *addr)
+{
+ unsigned long __percpu *a = (unsigned long *)addr + nr / BITS_PER_LONG;
+
+ return ((1UL << (nr % BITS_PER_LONG)) & percpu_read_stable(*a)) != 0;
+}
+
+static inline int this_cpu_variable_test_bit(int nr,
+ const unsigned long __percpu *addr)
+{
+ int oldbit;
+
+ asm volatile("bt "__percpu_arg(2)",%1\n\t"
+ "sbb %0,%0"
+ : "=r" (oldbit)
+ : "m" (*(unsigned long *)addr), "Ir" (nr));
+
+ return oldbit;
+}
+
+#define this_cpu_test_bit(nr, addr) \
+ (__builtin_constant_p((nr)) \
+ ? this_cpu_constant_test_bit((nr), (addr)) \
+ : this_cpu_variable_test_bit((nr), (addr)))
+
+
#include <asm-generic/percpu.h>

/* We can use this directly for local CPU (faster). */
Index: linux-2.6/arch/x86/kernel/process.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/process.c 2010-12-15 12:38:52.000000000 -0600
+++ linux-2.6/arch/x86/kernel/process.c 2010-12-15 12:38:53.000000000 -0600
@@ -445,7 +445,7 @@ void mwait_idle_with_hints(unsigned long
{
trace_power_start(POWER_CSTATE, (ax>>4)+1, smp_processor_id());
if (!need_resched()) {
- if (cpu_has(__this_cpu_ptr(&cpu_info), X86_FEATURE_CLFLUSH_MONITOR))
+ if (this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR))
clflush((void *)&current_thread_info()->flags);

__monitor((void *)&current_thread_info()->flags, 0, 0);
@@ -460,7 +460,7 @@ static void mwait_idle(void)
{
if (!need_resched()) {
trace_power_start(POWER_CSTATE, 1, smp_processor_id());
- if (cpu_has(__this_cpu_ptr(&cpu_info), X86_FEATURE_CLFLUSH_MONITOR))
+ if (this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR))
clflush((void *)&current_thread_info()->flags);

__monitor((void *)&current_thread_info()->flags, 0, 0);
Index: linux-2.6/arch/x86/kernel/smpboot.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/smpboot.c 2010-12-15 12:38:52.000000000 -0600
+++ linux-2.6/arch/x86/kernel/smpboot.c 2010-12-15 12:38:53.000000000 -0600
@@ -1397,9 +1397,9 @@ static inline void mwait_play_dead(void)
int i;
void *mwait_ptr;

- if (!cpu_has(__this_cpu_ptr(&cpu_info), X86_FEATURE_MWAIT))
+ if (!this_cpu_has(X86_FEATURE_MWAIT))
return;
- if (!cpu_has(__this_cpu_ptr(&cpu_info), X86_FEATURE_CLFLSH))
+ if (!this_cpu_has(X86_FEATURE_CLFLSH))
return;
if (__this_cpu_read(cpu_info.cpuid_level) < CPUID_MWAIT_LEAF)
return;
--
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