powerpc/64s: Dedicated system reset interrupt stack

The system reset interrupt is used for crash/debug situations, so it is
desirable to have as little impact on the normal state of the system as
possible.

Currently it uses the current kernel stack to process the exception.
This stores into the stack which may be involved with the crash. The
stack pointer may be corrupted, or it may have overflowed.

Avoid or minimise these problems by creating a dedicated NMI stack for
the system reset interrupt to use.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/paca.h

@@ -185,7 +185,8 @@ struct paca_struct {
 	u64 exmc[13];		/* used for machine checks */
 #endif
 #ifdef CONFIG_PPC_BOOK3S_64
-	/* Exclusive emergency stack pointer for machine check exception. */
+	/* Exclusive stacks for system reset and machine check exception. */
+	void *nmi_emergency_sp;
 	void *mc_emergency_sp;
 
 	u16 in_nmi;			/* In nmi handler */