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 <svaidy@linux.vnet.ibm.com>
Cc: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

3 files changed, 16 insertions, 5 deletions

diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index b24b7001ae19..bed9a29ee383 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/arch/powerpc/kernel/head_64.S
@@ -617,6 +617,17 @@ _GLOBAL(start_secondary_prolog)
         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
 
 /*
diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
index d1b124e44d77..349e0af9389a 100644
--- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
+++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
@@ -146,12 +146,11 @@ static void pseries_mach_cpu_die(void)
                 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) {
 
diff --git a/arch/powerpc/platforms/pseries/offline_states.h b/arch/powerpc/platforms/pseries/offline_states.h
index 202d8692c6a2..75a6f480d931 100644
--- a/arch/powerpc/platforms/pseries/offline_states.h
+++ b/arch/powerpc/platforms/pseries/offline_states.h
@@ -35,4 +35,5 @@ static inline void set_default_offline_state(int cpu)
 
 extern enum cpu_state_vals get_preferred_offline_state(int cpu);
 extern int start_secondary(void);
+extern void start_secondary_resume(void);
 #endif