[PATCH v4 13/27] x86,tracing: Add comments to do_nmi()

[Peter Zijlstra]

Add a few comments to do_nmi() as a result of the audit.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 arch/x86/kernel/nmi.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -529,11 +529,14 @@ do_nmi(struct pt_regs *regs, long error_
 	 * continue to use the NMI stack.
 	 */
 	if (unlikely(is_debug_stack(regs->sp))) {
-		debug_stack_set_zero();
+		debug_stack_set_zero(); /* notrace due to Makefile */
 		this_cpu_write(update_debug_stack, 1);
 	}
 #endif
 
+	/*
+	 * It is important that no tracing happens before nmi_enter()!
+	 */
 	nmi_enter();
 
 	inc_irq_stat(__nmi_count);
@@ -542,10 +545,13 @@ do_nmi(struct pt_regs *regs, long error_
 		default_do_nmi(regs);
 
 	nmi_exit();
+	/*
+	 * No tracing after nmi_exit()!
+	 */
 
 #ifdef CONFIG_X86_64
 	if (unlikely(this_cpu_read(update_debug_stack))) {
-		debug_stack_reset();
+		debug_stack_reset(); /* notrace due to Makefile */
 		this_cpu_write(update_debug_stack, 0);
 	}
 #endif