x86: Document the NMI handler about not using paranoid_exit

Linus cleaned up the NMI handler but it still needs some comments to
explain why it uses save_paranoid but not paranoid_exit. Just to keep
others from adding that in the future, document why it's not used.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

arch/x86/kernel/entry_64.S

@@ -1480,9 +1480,16 @@ END(error_exit)
 ENTRY(nmi)
 	INTR_FRAME
 	PARAVIRT_ADJUST_EXCEPTION_FRAME
-	pushq_cfi $-1
+	pushq_cfi $-1		/* ORIG_RAX: no syscall to restart */
 	subq $ORIG_RAX-R15, %rsp
 	CFI_ADJUST_CFA_OFFSET ORIG_RAX-R15
+	/*
+	 * Use save_paranoid to handle SWAPGS, but no need to use paranoid_exit
+	 * as we should not be calling schedule in NMI context.
+	 * Even with normal interrupts enabled. An NMI should not be
+	 * setting NEED_RESCHED or anything that normal interrupts and
+	 * exceptions might do.
+	 */
 	call save_paranoid
 	DEFAULT_FRAME 0
 	/* paranoidentry do_nmi, 0; without TRACE_IRQS_OFF */