Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753908Ab3JQJk0 (ORCPT ); Thu, 17 Oct 2013 05:40:26 -0400 Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:42953 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753137Ab3JQJkY (ORCPT ); Thu, 17 Oct 2013 05:40:24 -0400 Date: Thu, 17 Oct 2013 10:39:44 +0100 From: Will Deacon To: Jiang Liu Cc: Steven Rostedt , Catalin Marinas , Sandeepa Prabhu , Jiang Liu , Marc Zyngier , Arnd Bergmann , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v3 6/7] arm64, jump label: optimize jump label implementation Message-ID: <20131017093944.GB18765@mudshark.cambridge.arm.com> References: <1381893492-7135-1-git-send-email-liuj97@gmail.com> <1381893492-7135-7-git-send-email-liuj97@gmail.com> <20131016114608.GH5403@mudshark.cambridge.arm.com> <525EC8D1.7000900@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <525EC8D1.7000900@gmail.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1003 Lines: 23 On Wed, Oct 16, 2013 at 06:11:45PM +0100, Jiang Liu wrote: > On 10/16/2013 07:46 PM, Will Deacon wrote: > > > >> + } else { > >> + insn = aarch64_insn_gen_nop(); > > > > You could make the code more concise by limiting your patching ability to > > branch immediates. Then a nop is simply a branch to the next instruction (I > > doubt any modern CPUs will choke on this, whereas the architecture requires > > a NOP to take time). > I guess a NOP should be more effecient than a "B #4" on real CPUs:) Well, I was actually questioning that. A NOP *has* to take time (the architecture prevents implementations from discaring it) whereas a static, unconditional branch will likely be discarded early on by CPUs with even simple branch prediction logic. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/