linux-kernel - [PATCH] x86/asm: Use 32bit xor to clear registers

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Date: Wed, 24 Jan 2024 11:38:59 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH] x86/asm: Use 32bit xor to clear registers

x86_64 zero extends 32bit operations, so for 64bit operands,
XORL r32,r32 is functionally equal to XORQ r64,r64, but avoids
a REX prefix byte when legacy registers are used.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
---
 arch/x86/kernel/head_64.S         | 6 +++---
 arch/x86/kernel/sev_verify_cbit.S | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index cc3a81852e4a..ed287170c126 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -171,7 +171,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	ANNOTATE_NOENDBR
 
 	/* Clear %R15 which holds the boot_params pointer on the boot CPU */
-	xorq	%r15, %r15
+	xorl	%r15d, %r15d
 
 	/*
 	 * Retrieve the modifier (SME encryption mask if SME is active) to be
@@ -180,7 +180,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 #ifdef CONFIG_AMD_MEM_ENCRYPT
 	movq	sme_me_mask, %rax
 #else
-	xorq	%rax, %rax
+	xorl	%eax, %eax
 #endif
 
 	/* Form the CR3 value being sure to include the CR3 modifier */
@@ -297,7 +297,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 
 .Llookup_AP:
 	/* EAX contains the APIC ID of the current CPU */
-	xorq	%rcx, %rcx
+	xorl	%ecx, %ecx
 	leaq	cpuid_to_apicid(%rip), %rbx
 
 .Lfind_cpunr:
diff --git a/arch/x86/kernel/sev_verify_cbit.S b/arch/x86/kernel/sev_verify_cbit.S
index 3355e27c69eb..1ab65f6c6ae7 100644
--- a/arch/x86/kernel/sev_verify_cbit.S
+++ b/arch/x86/kernel/sev_verify_cbit.S
@@ -77,7 +77,7 @@ SYM_FUNC_START(sev_verify_cbit)
 	 * The check failed, prevent any forward progress to prevent ROP
 	 * attacks, invalidate the stack and go into a hlt loop.
 	 */
-	xorq	%rsp, %rsp
+	xorl	%esp, %esp
 	subq	$0x1000, %rsp
 2:	hlt
 	jmp 2b
-- 
2.31.1