powerpc: Reset kernel stack on cpu online from cede state

Cpu hotplug (offline) without dlpar operation will place cpu
in cede state and the extended_cede_processor() function will
return when resumed.

Kernel stack pointer needs to be reset before
start_secondary() is called to continue the online operation.

Added new function start_secondary_resume() to do the above
steps.

Signed-off-by: Vaidyanathan Srinivasan <[e-mail]>
Cc: Gautham R Shenoy <[e-mail]>
Signed-off-by: Benjamin Herrenschmidt <[e-mail]>

arch/powerpc/kernel/head_64.S

···
 	std	r3,0(r1)		/* Zero the stack frame pointer	*/
 	bl	.start_secondary
 	b	.
+/*
+ * Reset stack pointer and call start_secondary
+ * to continue with online operation when woken up
+ * from cede in cpu offline.
+ */
+_GLOBAL(start_secondary_resume)
+	ld	r1,PACAKSAVE(r13)	/* Reload kernel stack pointer */
+	li	r3,0
+	std	r3,0(r1)		/* Zero the stack frame pointer	*/
+	bl	.start_secondary
+	b	.
 #endif
 
 /*

arch/powerpc/platforms/pseries/hotplug-cpu.c

···
 		unregister_slb_shadow(hwcpu, __pa(get_slb_shadow()));
 
 		/*
-		 * NOTE: Calling start_secondary() here for now to
-		 * start new context.
-		 * However, need to do it cleanly by resetting the
-		 * stack pointer.
+		 * Call to start_secondary_resume() will not return.
+		 * Kernel stack will be reset and start_secondary()
+		 * will be called to continue the online operation.
 		 */
-		start_secondary();
+		start_secondary_resume();
 
 	} else if (get_preferred_offline_state(cpu) == CPU_STATE_OFFLINE) {
 

arch/powerpc/platforms/pseries/offline_states.h

···
 
 extern enum cpu_state_vals get_preferred_offline_state(int cpu);
 extern int start_secondary(void);
+extern void start_secondary_resume(void);
 #endif