[PATCH 1/2] x86/nmi: Optimize the check for being in the repeat_nmi code

[Steven Rostedt]

From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>

Linus mentioned that doing two compares can be replaced by a single
compare. That is, instead of:

   movq $repeat_nmi, %rdx
   cmpq 8(%rsp), %rdx
   ja not_in_region
   movq $end_repeat_nmi, %rdx
   cmpq 8(%rsp), %rdx
   ja in_region

we can replace that with:

   movq 8(%rsp), %rdx
   subq $repeat_nmi, %rdx
   cmpq $end_repeat_nmi-repeat_nmi, %rdx
   jb in_region

Inspired-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 arch/x86/entry/entry_64.S | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 044d18ebc43c..3aad759aace2 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1330,13 +1330,10 @@ ENTRY(nmi)
 	 * resume the outer NMI.
 	 */
 
-	movq	$repeat_nmi, %rdx
-	cmpq	8(%rsp), %rdx
-	ja	1f
-	movq	$end_repeat_nmi, %rdx
-	cmpq	8(%rsp), %rdx
-	ja	nested_nmi_out
-1:
+	movq	8(%rsp), %rdx
+	subq	$repeat_nmi, %rdx
+	cmpq	$end_repeat_nmi-repeat_nmi, %rdx
+	jb	nested_nmi_out
 
 	/*
 	 * Now check "NMI executing".  If it's set, then we're nested.
-- 
2.10.2