x86-32, hotplug: Add start_cpu0() entry point to head_32.S

start_cpu0() is defined in head_32.S for 32-bit. The function sets up stack and
jumps to start_secondary() for CPU0 wake up.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1352835171-3958-9-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/kernel/head_32.S

@@ -266,6 +266,19 @@ num_subarch_entries = (. - subarch_entries) / 4
 	jmp default_entry
 #endif /* CONFIG_PARAVIRT */
 
+#ifdef CONFIG_HOTPLUG_CPU
+/*
+ * Boot CPU0 entry point. It's called from play_dead(). Everything has been set
+ * up already except stack. We just set up stack here. Then call
+ * start_secondary().
+ */
+ENTRY(start_cpu0)
+	movl stack_start, %ecx
+	movl %ecx, %esp
+	jmp  *(initial_code)
+ENDPROC(start_cpu0)
+#endif
+
 /*
  * Non-boot CPU entry point; entered from trampoline.S
  * We can't lgdt here, because lgdt itself uses a data segment, but