Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759041AbXJOVjp (ORCPT ); Mon, 15 Oct 2007 17:39:45 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754632AbXJOVjh (ORCPT ); Mon, 15 Oct 2007 17:39:37 -0400 Received: from pentafluge.infradead.org ([213.146.154.40]:47067 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754570AbXJOVjg (ORCPT ); Mon, 15 Oct 2007 17:39:36 -0400 Date: Mon, 15 Oct 2007 14:37:32 -0700 From: Arjan van de Ven To: Mikulas Patocka Cc: Nick Piggin , Linux Kernel Mailing List Subject: Re: LFENCE instruction (was: [rfc][patch 3/3] x86: optimise barriers) Message-ID: <20071015143732.01d99af8@laptopd505.fenrus.org> In-Reply-To: References: Organization: Intel X-Mailer: Claws Mail 3.0.2 (GTK+ 2.12.0; i386-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-SRS-Rewrite: SMTP reverse-path rewritten from by pentafluge.infradead.org See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1263 Lines: 31 On Mon, 15 Oct 2007 22:47:42 +0200 (CEST) Mikulas Patocka wrote: > > According to latest memory ordering specification documents from > > Intel and AMD, both manufacturers are committed to in-order loads > > from cacheable memory for the x86 architecture. Hence, smp_rmb() > > may be a simple barrier. > > > > http://developer.intel.com/products/processor/manuals/318147.pdf > > http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/24593.pdf > > Hi > > I'm just wondering about one thing --- what is LFENCE instruction > good for? > > SFENCE is for enforcing ordering in write-combining buffers (it > doesn't have sense in write-back cache mode). > MFENCE is for preventing of moving stores past loads. > > But what is LFENCE for? I read the above documents and they already > say that CPUs have ordered loads. > The cpus also have an explicit set of instructions that deliberately do unordered stores/loads, and s/lfence etc are mostly designed for those. - 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/