From mboxrd@z Thu Jan 1 00:00:00 1970 Received: with ECARTIS (v1.0.0; list linux-mips); Wed, 04 Dec 2002 16:06:45 +0100 (CET) Received: from mx2.mips.com ([206.31.31.227]:15826 "EHLO mx2.mips.com") by linux-mips.org with ESMTP id ; Wed, 4 Dec 2002 16:06:44 +0100 Received: from newman.mips.com (ns-dmz [206.31.31.225]) by mx2.mips.com (8.12.5/8.12.5) with ESMTP id gB4F6TNf023205; Wed, 4 Dec 2002 07:06:29 -0800 (PST) Received: from copfs01.mips.com (copfs01 [192.168.205.101]) by newman.mips.com (8.9.3/8.9.0) with ESMTP id HAA18802; Wed, 4 Dec 2002 07:06:21 -0800 (PST) Received: from mips.com (copsun17 [192.168.205.27]) by copfs01.mips.com (8.11.4/8.9.0) with ESMTP id gB4F6Kb27587; Wed, 4 Dec 2002 16:06:21 +0100 (MET) Message-ID: <3DEE19EC.DD007304@mips.com> Date: Wed, 04 Dec 2002 16:06:20 +0100 From: Carsten Langgaard X-Mailer: Mozilla 4.77 [en] (X11; U; SunOS 5.8 sun4u) X-Accept-Language: en MIME-Version: 1.0 To: Ralf Baechle CC: linux-mips@linux-mips.org, dom@mips.com, chris@mips.com, kevink@mips.com Subject: Re: Prefetches in memcpy References: <3DC7CB8B.E2C1D4E5@mips.com> <20021105163806.A24996@bacchus.dhis.org> Content-Type: text/plain; charset=iso-8859-15 Content-Transfer-Encoding: 7bit Return-Path: X-Envelope-To: <"|/home/ecartis/ecartis -s linux-mips"> (uid 0) X-Orcpt: rfc822;linux-mips@linux-mips.org Original-Recipient: rfc822;linux-mips@linux-mips.org X-archive-position: 748 X-ecartis-version: Ecartis v1.0.0 Sender: linux-mips-bounce@linux-mips.org Errors-to: linux-mips-bounce@linux-mips.org X-original-sender: carstenl@mips.com Precedence: bulk X-list: linux-mips I would like to open this thread again, since nothing has been done about it. I think we should get rid of the prefetches until someone comes up with a version that doesn't prefetch beyond the copy destination/source area. /Carsten Ralf Baechle wrote: > On Tue, Nov 05, 2002 at 02:45:47PM +0100, Carsten Langgaard wrote: > > > The problem is the prefetches in the memcpy function in the kernel. > > There is spread a number of PREF instructions in the memcpy function, > > but there is no check if we are prefetching out-side the areas we are > > copying to/from. This is extremely dangerous because we might prefetch > > out-side the physical memory area, causing e.g. a bus error or something > > even more nasty. > > > > I recently found something even nastier, it could also hit a DMA buffer > > region, and thereby break the PCI DMA flushing scheme. > > For example if the kernel is doing a memcpy from an area that's next to > > a DMA buffer area, we could end up in a situation where, we are > > prefetching > > data into the cache from a memory location that is used for DMA transfer > > and owned by the device, but the DMA transfer has not yet completed. > > We then end up in a situation, where the memory and cache is out of sync > > and the cache is containing some old data. > > > > So we definitely need to do something about the prefetches in the memcpy > > function. We can either get rid of all the prefetches or make sure we > > don't prefetch out side the "memcpy" area. > > We could fix the prefetch into DMA buffer problem with an extra flush but > that's going to be expensive, I rather think we should avoid prefetches. > As Kevin explained KSEG1 is a loophole in the spec so we can't really say > what the behaviour of memcpy will be in KSEG1. > > So I think the fix will have to be: > > - Avoid prefetching beyond the end of the copy area in memcpy and memmove. > - Introduce a second variant of memcpy that never does prefetching. This > one will be safe to use in KSEG1 / uncached XKPHYS also and will be used > for memcpy_fromio, memcpy_toio and friends. > > Ralf -- _ _ ____ ___ Carsten Langgaard Mailto:carstenl@mips.com |\ /|||___)(___ MIPS Denmark Direct: +45 4486 5527 | \/ ||| ____) Lautrupvang 4B Switch: +45 4486 5555 TECHNOLOGIES 2750 Ballerup Fax...: +45 4486 5556 Denmark http://www.mips.com