From: Al Viro <viro@zeniv.linux.org.uk>
To: Uros Bizjak <ubizjak@gmail.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v2] x86: Use xorl %0,%0 in __get_user_asm
Date: Thu, 27 Aug 2020 21:14:23 +0100 [thread overview]
Message-ID: <20200827201423.GD1236603@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20200827180904.96399-1-ubizjak@gmail.com>
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...
next prev parent reply other threads:[~2020-08-27 20:14 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-27 18:09 [PATCH v2] x86: Use xorl %0,%0 in __get_user_asm Uros Bizjak
2020-08-27 18:55 ` H. Peter Anvin
2020-08-27 20:14 ` Al Viro [this message]
2020-08-28 6:41 ` Uros Bizjak
2020-09-03 20:57 ` [tip: x86/asm] x86/uaccess: Use XORL %0,%0 in __get_user_asm() tip-bot2 for Uros Bizjak
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