[Xen-devel] [PATCH] x86: optimize this_cpu()

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

Besides the .text space savings of over 2.5k on x86-64 (1.5k for
x86-32) this removes a load (plus a lea on x86-64) from various
frequently executed code paths, and finally provides a reason (other
than legibility) to prefer this_cpu() over per_cpu() in all places
where smp_processor_id() isn't being called anyway.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

--- 2010-06-15.orig/xen/include/asm-x86/current.h       2010-07-13 
14:38:21.000000000 +0200
+++ 2010-06-15/xen/include/asm-x86/current.h    2010-07-13 15:12:37.000000000 
+0200
@@ -17,6 +17,10 @@ struct vcpu;
 struct cpu_info {
     struct cpu_user_regs guest_cpu_user_regs;
     unsigned int         processor_id;
+    unsigned long        per_cpu_offset;
+#ifdef __x86_64__
+    unsigned long        __pad_for_stack_bottom;
+#endif
     struct vcpu         *current_vcpu;
 };
 
@@ -35,7 +39,12 @@ static inline struct cpu_info *get_cpu_i
 #define current               (get_current())
 
 #define get_processor_id()    (get_cpu_info()->processor_id)
-#define set_processor_id(id)  (get_cpu_info()->processor_id = (id))
+#define set_processor_id(id)  do {                                      \
+    struct cpu_info *ci__ = get_cpu_info();                             \
+    ci__->per_cpu_offset = __per_cpu_offset[ci__->processor_id = (id)]; \
+} while (0)
+
+#define get_per_cpu_offset()  (get_cpu_info()->per_cpu_offset)
 
 #define guest_cpu_user_regs() (&get_cpu_info()->guest_cpu_user_regs)
 
--- 2010-06-15.orig/xen/include/asm-x86/percpu.h        2010-07-13 
14:38:21.000000000 +0200
+++ 2010-06-15/xen/include/asm-x86/percpu.h     2010-07-13 13:56:38.000000000 
+0200
@@ -16,7 +16,7 @@ void percpu_init_areas(void);
 #define per_cpu(var, cpu)  \
     (*RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]))
 #define __get_cpu_var(var) \
-    (per_cpu(var, smp_processor_id()))
+    (*RELOC_HIDE(&per_cpu__##var, get_per_cpu_offset()))
 
 #define DECLARE_PER_CPU(type, name) extern __typeof__(type) per_cpu__##name
 



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