linux-kernel - [PATCH 1/1] x86/entry: Use one cmpq in NMI entry to check RIP for nested NMIs

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Date: Thu, 29 Feb 2024 09:37:11 +0100
From: Petr Tesarik <petrtesarik@...weicloud.com>
To: Andy Lutomirski <luto@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org (maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)),
	"H. Peter Anvin" <hpa@...or.com>,
	linux-kernel@...r.kernel.org (open list:X86 ENTRY CODE)
Cc: Roberto Sassu <roberto.sassu@...weicloud.com>,
	Petr Tesarik <petr.tesarik1@...wei-partners.com>
Subject: [PATCH 1/1] x86/entry: Use one cmpq in NMI entry to check RIP for nested NMIs

From: Petr Tesarik <petr.tesarik1@...wei-partners.com>

Optimize the check whether a nested NMI occurred between repeat_nmi and
end_repeat_nmi. Although this is not a hot path, this is standard code to
check whether a value is within a given range; it is slightly faster, takes
up less bytes of code and saves one entry in the branch predictor.

This patch also removes the only relocation for end_repeat_nmi, removing
the need for ANNOTATE_NOENDBR.

Signed-off-by: Petr Tesarik <petr.tesarik1@...wei-partners.com>
---
 arch/x86/entry/entry_64.S | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 9bb485977629..cae40076e109 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1251,13 +1251,10 @@ SYM_CODE_START(asm_exc_nmi)
 	 * 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.
@@ -1383,8 +1380,6 @@ repeat_nmi:
 	.endr
 	subq	$(5*8), %rsp
 end_repeat_nmi:
-	ANNOTATE_NOENDBR // this code
-
 	/*
 	 * Everything below this point can be preempted by a nested NMI.
 	 * If this happens, then the inner NMI will change the "iret"
-- 
2.34.1