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

To: "H. Peter Anvin" <hpa@xxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Thu, 06 Oct 2011 10:53:29 -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 05:17 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

Attachment: jmp2.c
Description: Text Data

Attachment: jmp5.c
Description: Text Data

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