FPU-intensive programs crashing with floating point exception on Cyrix MII

[Ondrej Zary <linux@rainbow-software.org>]

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My machine (Cyrix MII PR300 CPU, PCPartner TXB820DS board with i430TX 
chipset) exhibits a really weird problem:
When I run a program that uses FPU, it sometimes crashes with "flaoting 
point exception" - for example, when playing MP3 files using any player. 
Or with Prime95 - http://www.mersenne.org/freesoft.htm - the "torture 
test" does not crash but shows "fatal error" in less than 10 minutes.
It might be something like this:
http://lists.suse.com/archive/suse-linux-e/2000-Sep/1080.html
or this
http://lists.slug.org.au/archives/slug/2000/11/msg00343.html

The problem appears on 2.4.x kernels and 2.6.x kernels. It works fine in 
Windows 98 - it can play MP3s and run Prime95 for hours without any 
problems.
I've tracked it down to math_error() in arch/i386/kernel/traps.c and 
"fixed" it (I really don't know anything about FPU programming). The 
patch is attached. It fixes my system - with the patch, I can play MP3s 
fine and Prime95 runs without any problems too.

Does anyone know why these exceptions happen and/or what's the correct 
solution?

-- 
Ondrej Zary


[-- Attachment #2: cyrix-math.patch --]
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--- linux-2.6.10/arch/i386/kernel/traps.c~	2004-12-25 12:02:03.000000000 +0100
+++ linux-2.6.10/arch/i386/kernel/traps.c	2004-12-25 12:02:03.000000000 +0100
@@ -790,8 +790,11 @@
 	 */
 	cwd = get_fpu_cwd(task);
 	swd = get_fpu_swd(task);
+	printk("MATH ERROR %d\n",((~cwd) & swd & 0x3f) | (swd & 0x240));
 	switch (((~cwd) & swd & 0x3f) | (swd & 0x240)) {
-		case 0x000:
+		case 0x000: /* Hack for Cyrix problems */
+		case 0x200:
+			return;                                
 		default:
 			break;
 		case 0x001: /* Invalid Op */