From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935230Ab1JFHiV (ORCPT ); Thu, 6 Oct 2011 03:38:21 -0400 Received: from claw.goop.org ([74.207.240.146]:60716 "EHLO claw.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934406Ab1JFHiU (ORCPT ); Thu, 6 Oct 2011 03:38:20 -0400 Message-ID: <4E8CFA99.8030805@goop.org> Date: Wed, 05 Oct 2011 17:47:21 -0700 From: Jeremy Fitzhardinge User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:7.0.1) Gecko/20110930 Thunderbird/7.0.1 MIME-Version: 1.0 To: "H. Peter Anvin" CC: Jason Baron , Steven Rostedt , "David S. Miller" , David Daney , Michael Ellerman , Jan Glauber , the arch/x86 maintainers , Xen Devel , Linux Kernel Mailing List , Jeremy Fitzhardinge , peterz@infradead.org Subject: Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out References: <477dead9647029012f93c651f2892ed0e86b89e7.1317506051.git.jeremy.fitzhardinge@citrix.com> <20111003150205.GB2462@redhat.com> <4E89E28C.7010700@goop.org> <20111004141011.GA2520@redhat.com> <4E8B3489.60902@zytor.com> <4E8CF348.4080405@goop.org> <4E8CF385.2080804@zytor.com> In-Reply-To: <4E8CF385.2080804@zytor.com> X-Enigmail-Version: 1.3.2 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. I was thinking about making the jump-label stuff generate a small jmp if the offset is small (specifically "jmp; nop3", or perhaps "jmp; ud2a; nop" to make absolutely sure there's no speculation beyond the jmp), on the grounds that, while it probably doesn't matter for any modern Intel/AMD processor, it may help for others. But I couldn't find any concrete evidence to support it, and there's already enough questions about doing "live" code updates without adding more instruction patterns. J