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: <75651326-1d18-4db0-8995-ff4dd6dc1da3@broadcom.com>
Date: Wed, 7 Jan 2026 12:24:53 -0800
From: Alexey Makhalov <alexey.makhalov@...adcom.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>,
 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: Re: [PATCH RESEND 2/2] x86/paravirt: Use XOR r32,r32 to clear
 register in pv_vcpu_is_preempted()

On 1/5/26 1:39 AM, Uros Bizjak 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: Alexey Makhalov <alexey.makhalov@...adcom.com>