linux-kernel - Re: [PATCH v1 2/8] x86/entry/clearregs: Add infrastructure to clear registers

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Message-ID: <20180111201001.GN6718@tassilo.jf.intel.com>
Date:   Thu, 11 Jan 2018 12:10:01 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Andi Kleen <andi@...stfloor.org>, tglx@...utronix.de,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org, dwmw@...zon.co.uk, pjt@...gle.com,
        luto@...nel.org, peterz@...radead.org, thomas.lendacky@....com,
        tim.c.chen@...ux.intel.com, gregkh@...ux-foundation.org,
        dave.hansen@...el.com, jikos@...nel.org
Subject: Re: [PATCH v1 2/8] x86/entry/clearregs: Add infrastructure to clear
 registers

> > +	.macro CLEAR_ALL_REGS
> > +	CLEAR_R8_TO_R15
> > +	xorl %eax, %eax
> > +	xorl %ebx, %ebx
> 
> How come you use xorl vs xorq?

64bit always zero extends 32->64, and xorl is one byte
shorter because it doesn't need a REX prefix.

-Andi