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>

1 files changed, 8 insertions, 1 deletions

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 3819ea907339..d1d5434e7f6a 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/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 */