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

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Message-ID: <CAFULd4ai5mWTSB_9-G+brpJJ6_3VFtqdcFm-4YtoNbhskXNctA@mail.gmail.com>
Date:   Fri, 28 Aug 2020 08:41:18 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     X86 ML <x86@...nel.org>, LKML <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 10:14 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> 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...

Should I submit a v3 with the offending sentence removed, or could I
just ask a committer to remove it on the fly?

Uros.