Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934326Ab0GOSzP (ORCPT ); Thu, 15 Jul 2010 14:55:15 -0400 Received: from terminus.zytor.com ([198.137.202.10]:33957 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934137Ab0GOSzN (ORCPT ); Thu, 15 Jul 2010 14:55:13 -0400 Message-ID: <4C3F5972.2030600@zytor.com> Date: Thu, 15 Jul 2010 11:54:42 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.10) Gecko/20100621 Fedora/3.0.5-1.fc13 Thunderbird/3.0.5 MIME-Version: 1.0 To: Jeremy Fitzhardinge CC: Zachary Amsden , Glauber Costa , Thomas Gleixner , Avi Kivity , Linux Kernel Mailing List Subject: Re: [PATCH] x86: fix ordering constraints on crX read/writes References: <4C3E363B.7060804@goop.org> <4C3E5637.4010300@redhat.com> <4C3E5C8C.8000800@goop.org> <4C3F1C84.8080503@goop.org> In-Reply-To: <4C3F1C84.8080503@goop.org> 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 Content-Length: 1173 Lines: 26 On 07/15/2010 07:34 AM, Jeremy Fitzhardinge wrote: > On 07/14/2010 06:29 PM, H. Peter Anvin wrote: >> Yes, it will definitely NOT be pruned. I'm going to file a gcc documentation request to see if any of this is actually needed, though. There may also be a need for gcc to handle *inbound* general memory constraints. >> > > You mean "depends on all prior memory updates"? We have been relying on > "memory" to do that (barrier(), for example), but it would be nice to > explicitly confirm that's OK, or get something which is guaranteed to be OK. > No, we haven't. You're misunderstanding what a "memory" clobber does. A clobber affects the output side only, but doesn't inherently provide ordering on the input side. Apparently this is implicit in "asm volatile", which is a very important property. -hpa -- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf. -- 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/