x86: split loading percpu segments from loading gdt

Impact: split out a function, no functional change

Xen needs to be able to access percpu data from very early on.  For
various reasons, it cannot also load the gdt at that time.   It does,
however, have a pefectly functional gdt at that point, so there's no
pressing need to reload the gdt.

Split the function to load the segment registers off, so Xen can call
it directly.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

arch/x86/include/asm/processor.h

@@ -769,6 +769,7 @@ extern struct desc_ptr		early_gdt_descr;
 
 extern void cpu_set_gdt(int);
 extern void switch_to_new_gdt(int);
+extern void load_percpu_segment(int);
 extern void cpu_init(void);
 
 static inline unsigned long get_debugctlmsr(void)

arch/x86/kernel/cpu/common.c

@@ -253,6 +253,16 @@ static char __cpuinit *table_lookup_model(struct cpuinfo_x86 *c)
 
 __u32 cleared_cpu_caps[NCAPINTS] __cpuinitdata;
 
+void load_percpu_segment(int cpu)
+{
+#ifdef CONFIG_X86_32
+	loadsegment(fs, __KERNEL_PERCPU);
+#else
+	loadsegment(gs, 0);
+	wrmsrl(MSR_GS_BASE, (unsigned long)per_cpu(irq_stack_union.gs_base, cpu));
+#endif
+}
+
 /* Current gdt points %fs at the "master" per-cpu area: after this,
  * it's on the real one. */
 void switch_to_new_gdt(int cpu)
@@ -263,12 +273,8 @@ void switch_to_new_gdt(int cpu)
 	gdt_descr.size = GDT_SIZE - 1;
 	load_gdt(&gdt_descr);
 	/* Reload the per-cpu base */
-#ifdef CONFIG_X86_32
-	loadsegment(fs, __KERNEL_PERCPU);
-#else
-	loadsegment(gs, 0);
-	wrmsrl(MSR_GS_BASE, (unsigned long)per_cpu(irq_stack_union.gs_base, cpu));
-#endif
+
+	load_percpu_segment(cpu);
 }
 
 static struct cpu_dev *cpu_devs[X86_VENDOR_NUM] = {};