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

From: Jeremy Fitzhardinge
Date: Thu Oct 06 2011 - 14:15:18 EST

On 10/06/2011 11:10 AM, Jason Baron wrote:
> On Thu, Oct 06, 2011 at 10:53:29AM -0700, Jeremy Fitzhardinge wrote:
>> On 10/05/2011 05:17 PM, H. Peter Anvin wrote:
>>> 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.
>> Looks like jmp2 is about 5% faster than jmp5 on Sandybridge with this
>> benchmark.
>> But insignificant difference on Nehalem.
>> J
> It would be cool if we could make the total width 2-bytes, when
> possible. It might be possible by making the initial 'JUMP_LABEL_INITIAL_NOP'
> as a 'jmp' to the 'l_yes' label. And then patching that with a no-op at boot
> time or link time - letting the compiler pick the width. In that way we could
> get the optimal width...

I'll have a look at it later today if I get a moment. Should be fairly

What about the rest of the series. Do you think it looks cooked enough
for next mergewindow?

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