xtensa: implement fake NMI

In case perf IRQ is the highest of the medium-level IRQs, and is alone
on its level, it may be treated as NMI:
- LOCKLEVEL is defined to be one level less than EXCM level,
- IRQ masking never lowers current IRQ level,
- new fake exception cause code, EXCCAUSE_MAPPED_NMI is assigned to that
  IRQ; new second level exception handler, do_nmi, assigned to it
  handles it as NMI,
- atomic operations in configurations without s32c1i still need to mask
  all interrupts.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/irqflags.h

@@ -6,6 +6,7 @@
  * for more details.
  *
  * Copyright (C) 2001 - 2005 Tensilica Inc.
+ * Copyright (C) 2015 Cadence Design Systems Inc.
  */
 
 #ifndef _XTENSA_IRQFLAGS_H
@@ -23,8 +24,27 @@ static inline unsigned long arch_local_save_flags(void)
 static inline unsigned long arch_local_irq_save(void)
 {
 	unsigned long flags;
-	asm volatile("rsil %0, "__stringify(LOCKLEVEL)
+#if XTENSA_FAKE_NMI
+#if defined(CONFIG_DEBUG_KERNEL) && (LOCKLEVEL | TOPLEVEL) >= XCHAL_DEBUGLEVEL
+	unsigned long tmp;
+
+	asm volatile("rsr	%0, ps\t\n"
+		     "extui	%1, %0, 0, 4\t\n"
+		     "bgei	%1, "__stringify(LOCKLEVEL)", 1f\t\n"
+		     "rsil	%0, "__stringify(LOCKLEVEL)"\n"
+		     "1:"
+		     : "=a" (flags), "=a" (tmp) :: "memory");
+#else
+	asm volatile("rsr	%0, ps\t\n"
+		     "or	%0, %0, %1\t\n"
+		     "xsr	%0, ps\t\n"
+		     "rsync"
+		     : "=&a" (flags) : "a" (LOCKLEVEL) : "memory");
+#endif
+#else
+	asm volatile("rsil	%0, "__stringify(LOCKLEVEL)
 		     : "=a" (flags) :: "memory");
+#endif
 	return flags;
 }