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

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Message-ID: <E17D1D99-5F9C-4D49-9F4B-91AFE33E6EC4@zytor.com>
Date: Wed, 07 Jan 2026 01:54:40 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Uros Bizjak <ubizjak@...il.com>, bcm-kernel-feedback-list@...adcom.com,
        virtualization@...ts.linux.dev, x86@...nel.org,
        linux-kernel@...r.kernel.org
CC: 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>
Subject: Re: [PATCH RESEND 2/2] x86/paravirt: Use XOR r32,r32 to clear register in pv_vcpu_is_preempted()

On January 5, 2026 1:39:07 AM PST, Uros Bizjak <ubizjak@...il.com> wrote:
>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>
>Reviewed-by: 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));
> }
> 

Acked-by: H. Peter Anvin (Intel) <hpa@...or.com>