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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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