From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH RFC V2 3/5] jump_label: if a key has already
 been initialized, don't nop it out
Date: Tue, 04 Oct 2011 09:30:01 -0700
Message-ID: <4E8B3489.60902@zytor.com>
References: <cover.1317506051.git.jeremy.fitzhardinge@citrix.com>
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	<20111003150205.GB2462@redhat.com> <4E89E28C.7010700@goop.org>
	<20111004141011.GA2520@redhat.com>
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To: Jason Baron <jbaron@redhat.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>, the arch/x86 maintainers <x86@kernel.org>, Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>, Jan Glauber <jang@linux.vnet.ibm.com>, peterz@infradead.org, David Daney <david.daney@cavium.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Steven Rostedt <rostedt@goodmis.org>, Michael Ellerman <michael@ellerman.id.au>, Xen Devel <xen-devel@lists.xensource.com>, "David S. Miller" <davem@davemloft.net>

On 10/04/2011 07:10 AM, Jason Baron wrote:
> 
> 1) The jmp +0, is a 'safe' no-op that I know is going to initially
> boot for all x86. I'm not sure if there is a 5-byte nop that works on
> all x86 variants - but by using jmp +0, we make it much easier to debug
> cases where we may be using broken no-ops.
> 

There are *plenty*.  jmp+0 is about as pessimal as you can get.

The current recommendation when you don't know the CPU you're running at is:

	3E 8D 74 26 00	(GENERIC_NOP5_ATOMIC)

... on 32 bits and ...

	0F 1F 44 00 00	(P6_NOP5_ATOMIC)

... on 64 bits.

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.