Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758868AbYFKOqz (ORCPT ); Wed, 11 Jun 2008 10:46:55 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754716AbYFKOqo (ORCPT ); Wed, 11 Jun 2008 10:46:44 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:58707 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753461AbYFKOqn (ORCPT ); Wed, 11 Jun 2008 10:46:43 -0400 Date: Wed, 11 Jun 2008 07:46:03 -0700 (PDT) From: Linus Torvalds To: Nick Piggin cc: Paul Mackerras , Matthew Wilcox , Trent Piepho , Russell King , Benjamin Herrenschmidt , David Miller , linux-arch@vger.kernel.org, scottwood@freescale.com, linuxppc-dev@ozlabs.org, alan@lxorguk.ukuu.org.uk, linux-kernel@vger.kernel.org Subject: Re: MMIO and gcc re-ordering issue In-Reply-To: <200806111500.30789.nickpiggin@yahoo.com.au> Message-ID: References: <1211852026.3286.36.camel@pasglop> <200806101656.51211.nickpiggin@yahoo.com.au> <18511.21025.978328.596571@cargo.ozlabs.ibm.com> <200806111500.30789.nickpiggin@yahoo.com.au> User-Agent: Alpine 1.10 (LFD 962 2008-03-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1152 Lines: 31 On Wed, 11 Jun 2008, Nick Piggin wrote: > > I can't actually find the definitive statement in the Intel manuals > saying UC is strongly ordered also WRT WB. Linus? Definitive? Dunno. But look in the Architecture manual, volume 3A, 10.3 "Methods of Caching Available", and then under the bullet about Write Combining (WC), it says the writes may be delayed until the next occurrence of a serializing event; such as, an SFENCE of MFENCE instruction, CPUID execution, a read or write to uncached memory, an interrupt occurrence, or a LOCK instruction execution. However, it's worth noting that - documentation can be wrong, or even if right, can be Intel-specific. - the above is expressly _only_ about the WC buffer, not about regular memory writes. Cached memory accesses are different from WC accesses. so in the end, the thing that matters is how things actually work. Linus -- 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/