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

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Date:	Thu, 06 Oct 2011 11:15:07 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Jason Baron <jbaron@...hat.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"David S. Miller" <davem@...emloft.net>,
	David Daney <david.daney@...ium.com>,
	Michael Ellerman <michael@...erman.id.au>,
	Jan Glauber <jang@...ux.vnet.ibm.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	peterz@...radead.org, rth@...hat.com
Subject: Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized,
 don't nop it out

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
straightforward.

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

    J
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