powerpc: Machine check interrupt is a non-maskable interrupt

Use nmi_enter similarly to system reset interrupts. This uses NMI
printk NMI buffers and turns off various debugging facilities that
helps avoid tripping on ourselves or other CPUs.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/traps.c

@@ -644,8 +644,10 @@ int machine_check_generic(struct pt_regs *regs)
 
 void machine_check_exception(struct pt_regs *regs)
 {
-	enum ctx_state prev_state = exception_enter();
 	int recover = 0;
+	bool nested = in_nmi();
+	if (!nested)
+		nmi_enter();
 
 	/* 64s accounts the mce in machine_check_early when in HVMODE */
 	if (!IS_ENABLED(CONFIG_PPC_BOOK3S_64) || !cpu_has_feature(CPU_FTR_HVMODE))
@@ -677,10 +679,11 @@ void machine_check_exception(struct pt_regs *regs)
 
 	/* Must die if the interrupt is not recoverable */
 	if (!(regs->msr & MSR_RI))
-		panic("Unrecoverable Machine check");
+		nmi_panic(regs, "Unrecoverable Machine check");
 
 bail:
-	exception_exit(prev_state);
+	if (!nested)
+		nmi_exit();
 }
 
 void SMIException(struct pt_regs *regs)