[PATCH] powerpc/64s: watchdog fix stack setup

[Nicholas Piggin <npiggin@gmail.com>]

The watchdog soft-NMI exception stack setup loads a stack pointer
twice, which is an obvious error. It ends up using the system reset
interrupt (true-NMI) stack, which is also a bug because the watchdog
could be preempted by a system reset interrupt that overwrites the
NMI stack.

Change the soft-NMI to use the "emergency stack". The current kernel
stack is not used, because of the longer-term goal to prevent
asynchronous stack access using soft-disable.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---

This was tested by booting a kernel and verifying there was some
soft NMI activity, and also by deliberately causing a watchdog
lockup from the soft NMI path. Seems to be working.

In the system simulator you can inject a system reset when in the
soft_nmi_interrupt function and things go haywire without this
patch. 

 arch/powerpc/kernel/exceptions-64s.S | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index 9029afd1fa2a..f14f3c04ec7e 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S
@@ -1325,10 +1325,18 @@ EXC_VIRT_NONE(0x5800, 0x100)
 	std	r10,PACA_EXGEN+EX_R13(r13);		\
 	EXCEPTION_PROLOG_PSERIES_1(soft_nmi_common, _H)
 
+/*
+ * Branch to soft_nmi_interrupt using the emergency stack. The emergency
+ * stack is one that is usable by maskable interrupts so long as MSR_EE
+ * remains off. It is used for recovery when something has corrupted the
+ * normal kernel stack, for example. The "soft NMI" must not use the process
+ * stack because we want irq disabled sections to avoid touching the stack
+ * at all (other than PMU interrupts), so use the emergency stack for this,
+ * and run it entirely with interrupts hard disabled.
+ */
 EXC_COMMON_BEGIN(soft_nmi_common)
 	mr	r10,r1
 	ld	r1,PACAEMERGSP(r13)
-	ld	r1,PACA_NMI_EMERG_SP(r13)
 	subi	r1,r1,INT_FRAME_SIZE
 	EXCEPTION_COMMON_NORET_STACK(PACA_EXGEN, 0x900,
 			system_reset, soft_nmi_interrupt,
-- 
2.11.0