Monday, January 21, 2002
AMD Althon/Duron Bug -- 9:34 am CST, Update by A.T. Hun
There seems to be a bug in AMD's Athlon and Duron processors that causes lockups while using AGP in Linux. This is not a Linux-specific issue. AMD released a patch for Win2K back in September of 2000 for this. You can get the scoop on Gentoo Linux, which is currently being Slashdotted into oblivion. Here's a snip:As you may know, x86 systems have traditionally managed memory using 4K pages. However, with the introduction of the Pentium processor, Intel added a new feature called extended paging, which allows 4Mb pages to be used instead. Here's the problem -- many Athlon and Duron CPUs experience memory corruption when extended paging is used in conjunction with AGP. And, this problem hits us because Linux 2.4 kernels compiled with a Pentium-Classic or higher Processor family kernel configuration setting will automatically take advantage of extended paging (for kernel hackers out there, this is the X86_FEATURE_PSE constant defined in include/asm-i386/cpufeature.h.) Fortunately, there is a quick and easy fix for this problem. If you have been experiencing lockups on your Athlon, Duron or Athlon MP system when using AGP video, try passing the mem=nopentium option to your kernel (using GRUB or LILO) at boot-time. This tells Linux to go back to using 4K pages, avoiding this CPU bug. In addition, it should also be possible to avoid this problem by not using AGP on affected systems. As soon as I discovered that this CPU bug existed (which happened, unfortunately, because my CPU has the bug), I informed kernel hacker Andrew Morton of the issue; he put me in touch with Alan Cox. Alan is going to try to add some kind of Athlon/AGP CPU bug detection code to the kernel so that it will be able to auto-downgrade to 4K pages when necessary.Someone ran some Quake 3 benchmarks using that option. His framerate dropped from 79.4 to 79.2 so the perfomance hit barely registers. I wonder if this is part of the reason so many people complained about VIA chipsets locking up with AGP? This apparently affects every AMD processor, except the most recent revision of the Athlon XP, which includes this fix. I'm not sure how you can tell in advance if you have the newest version, other than by checking the CPUID. Check out AMD's white paper (PDF) on the model 6 (Athlon 4/XP/MP) revisions for more information.
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