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Date: Wed, 05 Oct 2011 17:17:09 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Jeremy Fitzhardinge <jeremy@goop.org>
CC: Jason Baron <jbaron@redhat.com>, Steven Rostedt <rostedt@goodmis.org>,
        "David S. Miller" <davem@davemloft.net>,
        David Daney <david.daney@cavium.com>,
        Michael Ellerman <michael@ellerman.id.au>,
        Jan Glauber <jang@linux.vnet.ibm.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Xen Devel <xen-devel@lists.xensource.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>,
        peterz@infradead.org
Subject: Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized,
 don't nop it out
References: <cover.1317506051.git.jeremy.fitzhardinge@citrix.com> <477dead9647029012f93c651f2892ed0e86b89e7.1317506051.git.jeremy.fitzhardinge@citrix.com> <20111003150205.GB2462@redhat.com> <4E89E28C.7010700@goop.org> <20111004141011.GA2520@redhat.com> <4E8B3489.60902@zytor.com> <4E8CF348.4080405@goop.org>
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On 10/05/2011 05:16 PM, Jeremy Fitzhardinge wrote:
> On 10/04/2011 09:30 AM, H. Peter Anvin wrote:
>> 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.
> 
> As an aside, do you know if a 2-byte unconditional jmp is any more
> efficient than 5-byte, aside from just being a smaller instruction and
> taking less icache?
> 

I don't know for sure, no.  I probably depends on the CPU.

	-hpa


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