powerpc: Mark system reset as an NMI with nmi_enter/exit()

System reset is a non-maskable interrupt from Linux's point of view
(occurs under local_irq_disable()), so it should use nmi_enter/exit.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/traps.c

@@ -279,6 +279,14 @@ void _exception(int signr, struct pt_regs *regs, int code, unsigned long addr)
 
 void system_reset_exception(struct pt_regs *regs)
 {
+	/*
+	 * Avoid crashes in case of nested NMI exceptions. Recoverability
+	 * is determined by RI and in_nmi
+	 */
+	bool nested = in_nmi();
+	if (!nested)
+		nmi_enter();
+
 	/* See if any machine dependent calls */
 	if (ppc_md.system_reset_exception) {
 		if (ppc_md.system_reset_exception(regs))
@@ -297,6 +305,9 @@ void system_reset_exception(struct pt_regs *regs)
 	if (!(regs->msr & MSR_RI))
 		panic("Unrecoverable System Reset");
 
+	if (!nested)
+		nmi_exit();
+
 	/* What should we do here? We could issue a shutdown or hard reset. */
 }