linux-kernel - Re: [PATCH/RFC] Math-emu kills the kernel on Athlon64 X2

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Date:	Mon, 2 Oct 2006 19:34:27 -0700 (PDT)
From:	Linus Torvalds <torvalds@...l.org>
To:	Randy Dunlap <rdunlap@...otime.net>
cc:	akpm <akpm@...l.org>, Jesper Juhl <jesper.juhl@...il.com>,
	Andi Kleen <ak@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC] Math-emu kills the kernel on Athlon64 X2



On Mon, 2 Oct 2006, Randy Dunlap wrote:
> 
> I had no trouble reproducing the boot failure (on Pentium-M), then
> I tried TRACE_RESUME().  Nifty, but not really needed here since
> earlyprintk worked and contained the fault messages:
> 
> [   16.841784] math_emulate: 0060:c01062dd
> [   16.845579] Kernel panic - not syncing: Math emulation needed in kernel
> 
> But CONFIG_MATH_EMULATION=y, so what now?

The "Math emulation needed in kernel" message means that it was asked to 
emulate a kernel instruction, and it refuses to do so. The emulation is 
_not_ meant to be a real FPU, it simply looks like one to user space. A 
lot of things aren't really emulated (there's no global x87 context, for 
example: the context is all strictly per-process).

> Linus mentioned CPU feature bits.  The message log above didn't
> make me feel good about them.  Sure enough, we are playing with
> features before reading the feature bits.

Please look up address c01062dd in the system map (or just using gdb), 
that will tell you what code _tried_ to use the math coprocessor in kernel 
space.

			Linus
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