Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757278AbcCCSf6 (ORCPT ); Thu, 3 Mar 2016 13:35:58 -0500 Received: from mx1.redhat.com ([209.132.183.28]:56218 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754762AbcCCSf4 (ORCPT ); Thu, 3 Mar 2016 13:35:56 -0500 Date: Thu, 3 Mar 2016 20:35:50 +0200 From: "Michael S. Tsirkin" To: Peter Zijlstra Cc: Ingo Molnar , Dexuan Cui , "linux-x86_64@vger.kernel.org" , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , David Howells , "Paul E. McKenney" , "linux-kernel@vger.kernel.org" Subject: Re: x86 memory barrier: why does Linux prefer MFENCE to Locked ADD? Message-ID: <20160303203414-mutt-send-email-mst@redhat.com> References: <20160303152739.GA16303@gmail.com> <20160303153453.GR6356@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20160303153453.GR6356@twins.programming.kicks-ass.net> X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Thu, 03 Mar 2016 18:35:55 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2361 Lines: 60 On Thu, Mar 03, 2016 at 04:34:53PM +0100, Peter Zijlstra wrote: > On Thu, Mar 03, 2016 at 04:27:39PM +0100, Ingo Molnar wrote: > > > > * Dexuan Cui wrote: > > > > > Hi, > > > My understanding about arch/x86/include/asm/barrier.h is: obviously Linux > > > more likes {L,S,M}FENCE -- Locked ADD is only used in x86_32 platforms that > > > don't support XMM2. > > > > > > However, it looks people say Locked Add is much faster than the FENCE > > > instructions, even on modern Intel CPUs like Haswell, e.g., please see > > > the three sources: > > > > > > " 11.5.1 Locked Instructions as Memory Barriers > > > Optimization > > > Use locked instructions to implement Store/Store and Store/Load barriers. > > > " > > > http://support.amd.com/TechDocs/47414_15h_sw_opt_guide.pdf > > > > > > "lock addl %(rsp), 0 is a better solution for StoreLoad barrier ": > > > http://shipilev.net/blog/2014/on-the-fence-with-dependencies/ > > > > > > "...locked instruction are more efficient barriers...": > > > http://www.pvk.ca/Blog/2014/10/19/performance-optimisation-~-writing-an-essay/ > > > > > > I also found that FreeBSD prefers Locked Add. > > > > > > So, I'm curious why Linux prefers MFENCE. > > > I guess I may be missing something. > > > > > > I tried to google the question, but didn't find an answer. > > > > It's being worked on, see this thread on lkml from a few weeks ago: > > > > C Jan 13 Michael S. Tsir | [PATCH v3 0/4] x86: faster mb()+documentation tweaks > > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 1/4] x86: add cc clobber for addl > > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 2/4] x86: drop a comment left over from X86_OOSTORE > > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 3/4] x86: tweak the comment about use of wmb for IO > > C Jan 13 Michael S. Tsir | ├─>[PATCH v3 4/4] x86: drop mfence in favor of lock+addl > > > > The 4th patch changes MFENCE to a LOCK ADDL locked instruction. > > Lots of additional chatter here: > > lkml.kernel.org/r/20160112150032-mutt-send-email-mst@redhat.com > > And some useful bits here: > > lkml.kernel.org/r/56957D54.5000602@zytor.com > > latest version here: > > lkml.kernel.org/r/1453921746-16178-1-git-send-email-mst@redhat.comZ It's ready as far as I am concerned. Basically we are just waiting for ack from hpa. -- MST