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SubjectRe: [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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