x86/cpu: Factor out common CPU initialization code, fix 32-bit Xen PV guests

Some of x86 bare-metal and Xen CPU initialization code is common
between the two and therefore can be factored out to avoid code
duplication.

As a side effect, doing so will also extend the fix provided by
commit a7fcf28d431e ("x86/asm/entry: Replace this_cpu_sp0() with
current_top_of_stack() to x86_32") to 32-bit Xen PV guests.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: konrad.wilk@oracle.com
Cc: xen-devel@lists.xenproject.org
Link: http://lkml.kernel.org/r/1427897534-5086-1-git-send-email-boris.ostrovsky@oracle.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/smp.h

@@ -154,6 +154,7 @@ void cpu_die_common(unsigned int cpu);
 void native_smp_prepare_boot_cpu(void);
 void native_smp_prepare_cpus(unsigned int max_cpus);
 void native_smp_cpus_done(unsigned int max_cpus);
+void common_cpu_up(unsigned int cpunum, struct task_struct *tidle);
 int native_cpu_up(unsigned int cpunum, struct task_struct *tidle);
 int native_cpu_disable(void);
 void native_cpu_die(unsigned int cpu);

arch/x86/kernel/smpboot.c

@@ -779,6 +779,26 @@ out:
 	return boot_error;
 }
 
+void common_cpu_up(unsigned int cpu, struct task_struct *idle)
+{
+	/* Just in case we booted with a single CPU. */
+	alternatives_enable_smp();
+
+	per_cpu(current_task, cpu) = idle;
+
+#ifdef CONFIG_X86_32
+	/* Stack for startup_32 can be just as for start_secondary onwards */
+	irq_ctx_init(cpu);
+	per_cpu(cpu_current_top_of_stack, cpu) =
+		(unsigned long)task_stack_page(idle) + THREAD_SIZE;
+#else
+	clear_tsk_thread_flag(idle, TIF_FORK);
+	initial_gs = per_cpu_offset(cpu);
+#endif
+	per_cpu(kernel_stack, cpu) =
+		(unsigned long)task_stack_page(idle) + THREAD_SIZE;
+}
+
 /*
  * NOTE - on most systems this is a PHYSICAL apic ID, but on multiquad
  * (ie clustered apic addressing mode), this is a LOGICAL apic ID.
@@ -796,24 +816,9 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
 	int cpu0_nmi_registered = 0;
 	unsigned long timeout;
 
-	/* Just in case we booted with a single CPU. */
-	alternatives_enable_smp();
-
 	idle->thread.sp = (unsigned long) (((struct pt_regs *)
 			  (THREAD_SIZE +  task_stack_page(idle))) - 1);
-	per_cpu(current_task, cpu) = idle;
 
-#ifdef CONFIG_X86_32
-	/* Stack for startup_32 can be just as for start_secondary onwards */
-	irq_ctx_init(cpu);
-	per_cpu(cpu_current_top_of_stack, cpu) =
-		(unsigned long)task_stack_page(idle) + THREAD_SIZE;
-#else
-	clear_tsk_thread_flag(idle, TIF_FORK);
-	initial_gs = per_cpu_offset(cpu);
-#endif
-	per_cpu(kernel_stack, cpu) =
-		(unsigned long)task_stack_page(idle) + THREAD_SIZE;
 	early_gdt_descr.address = (unsigned long)get_cpu_gdt_table(cpu);
 	initial_code = (unsigned long)start_secondary;
 	stack_start  = idle->thread.sp;
@@ -954,6 +959,8 @@ int native_cpu_up(unsigned int cpu, struct task_struct *tidle)
 	/* the FPU context is blank, nobody can own it */
 	__cpu_disable_lazy_restore(cpu);
 
+	common_cpu_up(cpu, tidle);
+
 	err = do_boot_cpu(apicid, cpu, tidle);
 	if (err) {
 		pr_err("do_boot_cpu failed(%d) to wakeup CPU#%u\n", err, cpu);

arch/x86/xen/smp.c

@@ -445,14 +445,7 @@ static int xen_cpu_up(unsigned int cpu, struct task_struct *idle)
 {
 	int rc;
 
-	per_cpu(current_task, cpu) = idle;
-#ifdef CONFIG_X86_32
-	irq_ctx_init(cpu);
-#else
-	clear_tsk_thread_flag(idle, TIF_FORK);
-#endif
-	per_cpu(kernel_stack, cpu) =
-		(unsigned long)task_stack_page(idle) + THREAD_SIZE;
+	common_cpu_up(cpu, idle);
 
 	xen_setup_runstate_info(cpu);
 	xen_setup_timer(cpu);
@@ -467,10 +460,6 @@ static int xen_cpu_up(unsigned int cpu, struct task_struct *idle)
 	if (rc)
 		return rc;
 
-	if (num_online_cpus() == 1)
-		/* Just in case we booted with a single CPU. */
-		alternatives_enable_smp();
-
 	rc = xen_smp_intr_init(cpu);
 	if (rc)
 		return rc;