perf, x86: Use ALTERNATIVE() to check for X86_FEATURE_PERFCTR_CORE

Using ALTERNATIVE() when checking for X86_FEATURE_PERFCTR_CORE avoids
an extra pointer chase and data cache hit.

Signed-off-by: Robert Richter <robert.richter@amd.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1302913676-14352-4-git-send-email-robert.richter@amd.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 11 insertions, 4 deletions

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index eed3673a865..224a84f7080 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -31,6 +31,7 @@
 #include <asm/nmi.h>
 #include <asm/compat.h>
 #include <asm/smp.h>
+#include <asm/alternative.h>
 
 #if 0
 #undef wrmsrl
@@ -363,12 +364,18 @@ again:
         return new_raw_count;
 }
 
-/* using X86_FEATURE_PERFCTR_CORE to later implement ALTERNATIVE() here */
 static inline int x86_pmu_addr_offset(int index)
 {
-        if (boot_cpu_has(X86_FEATURE_PERFCTR_CORE))
-                return index << 1;
-        return index;
+        int offset;
+
+        /* offset = X86_FEATURE_PERFCTR_CORE ? index << 1 : index */
+        alternative_io(ASM_NOP2,
+                       "shll $1, %%eax",
+                       X86_FEATURE_PERFCTR_CORE,
+                       "=a" (offset),
+                       "a"  (index));
+
+        return offset;
 }
 
 static inline unsigned int x86_pmu_config_addr(int index)