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Subject: Re: [PATCH v2] x86: Use xorl %0,%0 in __get_user_asm
To:     Uros Bizjak <ubizjak@gmail.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>
References: <20200827180904.96399-1-ubizjak@gmail.com>
From:   "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <53b5c10f-122f-3fb1-f371-7e1868f7384a@zytor.com>
Date:   Thu, 27 Aug 2020 11:55:05 -0700
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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@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: "H. Peter Anvin" <hpa@zytor.com>

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