linux-kernel - [PATCH 2/2] x86/paravirt: Use XOR r32,r32 to clear register in pv_vcpu_is

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Message-ID: <20251120191716.124642-2-ubizjak@gmail.com>
Date: Thu, 20 Nov 2025 20:15:57 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: bcm-kernel-feedback-list@...adcom.com,
	virtualization@...ts.linux.dev,
	x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Juergen Gross <jgross@...e.com>,
	Ajay Kaher <ajay.kaher@...adcom.com>,
	Alexey Makhalov <alexey.makhalov@...adcom.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	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 2/2] x86/paravirt: Use XOR r32,r32 to clear register in pv_vcpu_is_preempted()

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

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Juergen Gross <jgross@...e.com>
Cc: Ajay Kaher <ajay.kaher@...adcom.com>
Cc: Alexey Makhalov <alexey.makhalov@...adcom.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
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/include/asm/paravirt.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 4f6ec60b4cb3..59aec695ae5f 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -577,7 +577,7 @@ static __always_inline void pv_kick(int cpu)
 static __always_inline bool pv_vcpu_is_preempted(long cpu)
 {
 	return PVOP_ALT_CALLEE1(bool, lock.vcpu_is_preempted, cpu,
-				"xor %%" _ASM_AX ", %%" _ASM_AX,
+				"xor %%eax, %%eax",
 				ALT_NOT(X86_FEATURE_VCPUPREEMPT));
 }
 
-- 
2.51.1