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

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Message-ID: <20200827201423.GD1236603@ZenIV.linux.org.uk>
Date:   Thu, 27 Aug 2020 21:14:23 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2] x86: Use xorl %0,%0 in __get_user_asm

On Thu, Aug 27, 2020 at 08:09:04PM +0200, 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.

No objections, but talking about stalls is more than slightly
ridiculous - we'd just taken a #PF, failed there, flipped
pt_regs %rip to fixup section, returned from fault and are
about to fail whatever syscall that had been; a stall here
is really not an issue...