Linux-kernel mailing list archive 1998-31,: Re: floating-point abuse in 2.1.113

# Re: floating-point abuse in 2.1.113

Richard B. Johnson (root@chaos.analogic.com)
Tue, 4 Aug 1998 09:14:54 -0400 (EDT)

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On Tue, 4 Aug 1998, Samuli Valo wrote:

> On Tue, 4 Aug 1998, Mikael Pettersson wrote:
>
> > Right. Floating-point is banned within the Linux kernel, yet some
>
> Uh... Just a stupid question... Why floating-point is banned?
>

The state of the FPU is undefined within the kernel. You would have
to add a 'FINIT' instruction before the actual FP code (the compiler
won't do it for you). Further, if you have a FP fault that traps
to the kernel's FP fault-handler, you probably generated a double-fault
from which you can't recover. Further, if you use FP emulation such
as for the '386, every ESCAPE (preamble for the FP code) generates
a FP trap to the kernel emulator.

The undefined state comes about because, although FPU state is saved and
then restored as part of the context-switch, the FPU is left as it was.
It might have been calculating a FYL2XP1 (y * log2(x+1)) instruction
which is interuptable, then the kernel code pushes two operands onto
the FP unit's stack to perform a multiply. Not good.

Since the only "need" for float would be to perform some sort of
scaling, fixed-point integers will always work.

Cheers,
Dick Johnson
***** FILE SYSTEM MODIFIED *****
Penguin : Linux version 2.1.113 on an i586 machine (66.15 BogoMips).
Warning : It's hard to remain at the trailing edge of technology.

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