From owner-freebsd-current Thu Jan 31 16:19:47 2002 Delivered-To: freebsd-current@freebsd.org Received: from albatross.prod.itd.earthlink.net (albatross.mail.pas.earthlink.net [207.217.120.120]) by hub.freebsd.org (Postfix) with ESMTP id A1E5637B404; Thu, 31 Jan 2002 16:19:40 -0800 (PST) Received: from pool0167.cvx40-bradley.dialup.earthlink.net ([216.244.42.167] helo=mindspring.com) by albatross.prod.itd.earthlink.net with esmtp (Exim 3.33 #1) id 16WRQN-00068q-00; Thu, 31 Jan 2002 16:19:27 -0800 Message-ID: <3C59DF0A.8F850B66@mindspring.com> Date: Thu, 31 Jan 2002 16:19:22 -0800 From: Terry Lambert X-Mailer: Mozilla 4.7 [en]C-CCK-MCD {Sony} (Win98; U) X-Accept-Language: en MIME-Version: 1.0 To: Kenneth Culver Cc: David Malone , "Cameron, Frank" , "'freebsd-bugs@freebsd.org'" , "'freebsd-current@freebsd.org'" Subject: Re: AMD AGP Bug References: <20020131103927.A21783-100000@alpha.yumyumyum.org> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: owner-freebsd-current@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG Kenneth Culver wrote: > > There's actually a seperate TLB bug, but FreeBSD doesn't > > trigger that one, either (Linux can tickle it, when there > > are certain specific circumstances met). > > Well, I think I know what you're talking about, linux allocates agpgart > memory without setting a "non-cacheable" bit, and then the agp card writes > to that memory, but the cpu cached it already, which makes the cache wrong > or something like that, and causes the crashes/hangs. I know this is a > greatly simplified version of the real problem, but I think this is a > linux bug not necesarily an amd bug. No, you are describing the coherency bug in the chipset used for the AGP with AMD processors, which should enforce a coherency cycle, and cache invalidation for AGP based writes of the shared memory region. They are treating it as if it were non-cacheable dual ported SDRAM. There *really* is a bug in P4 and above and in late model AMD chips with 4M pages. It is very easy to reproduce with a few small kernel modifications (to actively use 4M pages) and 256M-512M of RAM. I'll characterize it further and provide the software workaround, for $10,000. It's about 16 machine instructions, if you write it out as code. Meanwhile, Linux and Windows can keep turning off the PSE, and losing 4-14% of their performance, depending on their application. -- Terry To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message