From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ozlabs.org ([203.10.76.45]:9098 "EHLO ozlabs.org") by vger.kernel.org with ESMTP id S932464AbWCHWK6 (ORCPT ); Wed, 8 Mar 2006 17:10:58 -0500 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-ID: <17423.21589.385336.68518@cargo.ozlabs.ibm.com> Date: Thu, 9 Mar 2006 09:01:57 +1100 From: Paul Mackerras Subject: Re: [PATCH] Document Linux's memory barriers [try #2] In-Reply-To: <20060308145506.GA5095@devserv.devel.redhat.com> References: <31492.1141753245@warthog.cambridge.redhat.com> <29826.1141828678@warthog.cambridge.redhat.com> <20060308145506.GA5095@devserv.devel.redhat.com> Sender: linux-arch-owner@vger.kernel.org To: Alan Cox Cc: David Howells , torvalds@osdl.org, akpm@osdl.org, mingo@redhat.com, linux-arch@vger.kernel.org, linuxppc64-dev@ozlabs.org, linux-kernel@vger.kernel.org List-ID: Alan Cox writes: > On Wed, Mar 08, 2006 at 02:37:58PM +0000, David Howells wrote: > > + (*) reads can be done speculatively, and then the result discarded should it > > + prove not to be required; > > That might be worth an example with an if() because PPC will do this and if > its a read with a side effect (eg I/O space) you get singed.. On PPC machines, the PTE has a bit called G (for Guarded) which indicates that the memory mapped by it has side effects. It prevents the CPU from doing speculative accesses (i.e. the CPU can't send out a load from the page until it knows for sure that the program will get to that instruction) and from prefetching from the page. The kernel sets G=1 on MMIO and PIO pages in general, as you would expect, although you can get G=0 mappings for framebuffers etc. if you ask specifically for that. Paul.