From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758815Ab1JFSLT (ORCPT ); Thu, 6 Oct 2011 14:11:19 -0400 Received: from mx1.redhat.com ([209.132.183.28]:31971 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751842Ab1JFSLS (ORCPT ); Thu, 6 Oct 2011 14:11:18 -0400 Date: Thu, 6 Oct 2011 14:10:55 -0400 From: Jason Baron To: Jeremy Fitzhardinge Cc: "H. Peter Anvin" , 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, rth@redhat.com Subject: Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out Message-ID: <20111006181055.GA2505@redhat.com> 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> <4E8DEB19.1050509@goop.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4E8DEB19.1050509@goop.org> User-Agent: Mutt/1.5.20 (2009-12-10) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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... thanks, -Jason > #include > > struct { > unsigned char flag; > unsigned char val; > } l; > > #define JMP2 asm volatile ("jmp 1f; .byte 0x0f,0x1f,0x00; 1: "); > #define JMPJMP2 JMP2 JMP2 > #define JMPJMPJMP2 JMPJMP2 JMPJMP2 > #define JMPJMPJMP2 JMPJMP2 JMPJMP2 > #define JMPJMPJMPJMP2 JMPJMPJMP2 JMPJMPJMP2 > #define JMPJMPJMPJMPJMP2 JMPJMPJMPJMP2 JMPJMPJMPJMP2 > #define JMPJMPJMPJMPJMPJMP2 JMPJMPJMPJMPJMP2 JMPJMPJMPJMPJMP2 > > int main(int argc, char **argv) > { > int i; > > for (i = 0; i < 100000000; i++) { > JMPJMPJMPJMPJMPJMP2; > asm volatile("" : : : "memory"); > } > > return 0; > } > #include > > struct { > unsigned char flag; > unsigned char val; > } l; > > #define JMP5 asm volatile (".byte 0xe9; .long 0"); > #define JMPJMP5 JMP5 JMP5 > #define JMPJMPJMP5 JMPJMP5 JMPJMP5 > #define JMPJMPJMP5 JMPJMP5 JMPJMP5 > #define JMPJMPJMPJMP5 JMPJMPJMP5 JMPJMPJMP5 > #define JMPJMPJMPJMPJMP5 JMPJMPJMPJMP5 JMPJMPJMPJMP5 > #define JMPJMPJMPJMPJMPJMP5 JMPJMPJMPJMPJMP5 JMPJMPJMPJMPJMP5 > > int main(int argc, char **argv) > { > int i; > > for (i = 0; i < 100000000; i++) { > JMPJMPJMPJMPJMPJMP5; > asm volatile("" : : : "memory"); > } > > return 0; > }