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[Xen-devel] Re: [PATCH RFC V2 3/5] jump_label: if a key has already been

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: [Xen-devel] Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out
From: "H. Peter Anvin" <hpa@xxxxxxxxx>
Date: Wed, 05 Oct 2011 17:17:09 -0700
Cc: the arch/x86 maintainers <x86@xxxxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>, Jan Glauber <jang@xxxxxxxxxxxxxxxxxx>, peterz@xxxxxxxxxxxxx, Jason Baron <jbaron@xxxxxxxxxx>, David Daney <david.daney@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Steven Rostedt <rostedt@xxxxxxxxxxx>, Michael Ellerman <michael@xxxxxxxxxxxxxx>, Xen Devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, "David S. Miller" <davem@xxxxxxxxxxxxx>
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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.


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