Date: Thu, 31 Jan 2002 16:19:22 -0800 From: Terry Lambert <tlambert2@mindspring.com> To: Kenneth Culver <culverk@yumyumyum.org> Cc: David Malone <dwmalone@maths.tcd.ie>, "Cameron, Frank" <Cameron@ctc.com>, "'freebsd-bugs@freebsd.org'" <freebsd-bugs@FreeBSD.ORG>, "'freebsd-current@freebsd.org'" <freebsd-current@FreeBSD.ORG> Subject: Re: AMD AGP Bug Message-ID: <3C59DF0A.8F850B66@mindspring.com> References: <20020131103927.A21783-100000@alpha.yumyumyum.org>
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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-bugs" in the body of the message
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