linux-kernel - Re: [PATCH v2] x86: Use xorl %0,%0 in __get_user

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Date:   Thu, 27 Aug 2020 11:55:05 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Uros Bizjak <ubizjak@...il.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v2] x86: Use xorl %0,%0 in __get_user_asm

On 2020-08-27 11:09, Uros Bizjak wrote:
> xorl %0,%0 is equivalent to xorq %0,%0 as both will zero the
> entire register.  Use xorl %0,%0 for all operand sizes to avoid
> REX prefix byte when legacy registers are used and to avoid size
> prefix byte when 16bit registers are used.
> 
> Zeroing the full register is OK in this use case.  xorl %0,%0 also
> breaks register dependency chains, avoiding potential partial
> register stalls with 8 and 16bit operands.
> 
> The patch lowers the size of .fixup section by 20 bytes.
> 
> Changes since v1:
> - Rewrite commit message.
> 
> 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: "H. Peter Anvin" <hpa@...or.com>

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