linux-kernel - [PATCH 1/2] KVM: VMX: Drop obsolete branch hint prefixes from inline asm

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20260211102928.100944-1-ubizjak@gmail.com>
Date: Wed, 11 Feb 2026 11:28:49 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: kvm@...r.kernel.org,
	x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH 1/2] KVM: VMX: Drop obsolete branch hint prefixes from inline asm

Remove explicit branch hint prefixes (.byte 0x2e / 0x3e) from VMX
inline assembly sequences.

These prefixes (CS/DS segment overrides used as branch hints on
very old x86 CPUs) have been ignored by modern processors for a
long time. Keeping them provides no measurable benefit and only
enlarges the generated code.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Thomas Gleixner <tglx@...nel.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
---
 arch/x86/kvm/vmx/vmx_ops.h | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx_ops.h b/arch/x86/kvm/vmx/vmx_ops.h
index 96677576c836..1000d37f5b0c 100644
--- a/arch/x86/kvm/vmx/vmx_ops.h
+++ b/arch/x86/kvm/vmx/vmx_ops.h
@@ -119,7 +119,6 @@ static __always_inline unsigned long __vmcs_readl(unsigned long field)
 #else /* !CONFIG_CC_HAS_ASM_GOTO_OUTPUT */
 
 	asm volatile("1: vmread %[field], %[output]\n\t"
-		     ".byte 0x3e\n\t" /* branch taken hint */
 		     "ja 3f\n\t"
 
 		     /*
@@ -191,7 +190,6 @@ static __always_inline unsigned long vmcs_readl(unsigned long field)
 #define vmx_asm1(insn, op1, error_args...)				\
 do {									\
 	asm goto("1: " __stringify(insn) " %0\n\t"			\
-			  ".byte 0x2e\n\t" /* branch not taken hint */	\
 			  "jna %l[error]\n\t"				\
 			  _ASM_EXTABLE(1b, %l[fault])			\
 			  : : op1 : "cc" : error, fault);		\
@@ -208,7 +206,6 @@ fault:									\
 #define vmx_asm2(insn, op1, op2, error_args...)				\
 do {									\
 	asm goto("1: "  __stringify(insn) " %1, %0\n\t"			\
-			  ".byte 0x2e\n\t" /* branch not taken hint */	\
 			  "jna %l[error]\n\t"				\
 			  _ASM_EXTABLE(1b, %l[fault])			\
 			  : : op1, op2 : "cc" : error, fault);		\
-- 
2.53.0