[Xen-devel] Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out

["H. Peter Anvin" <hpa@xxxxxxxxx>]

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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