Subject: Re: Floating point in the kernel
To: Chris G. Demetriou <>
From: Brian C. Grayson <>
List: tech-kern
Date: 09/18/1998 14:49:29
On Fri, Sep 18, 1998 at 08:52:34AM -0700, Chris G. Demetriou wrote:
> (Andreas Gustafsson) writes:
> > So, I am wondering, is this lack of support for floating point in the
> > kernel on the i386 port
> > 
> >  a) considered a bug or a feature?
> Feature.  It's a performance issue:
> If you can use FP in the kernel, then unless you're very careful about
> saves and restores around the places where you want to do FP work, you
> need to save FP state in every interrupt and trap entry and exit
> routine.

  I don't know what I'm talking about :), but since the use of FP
is going to be rare, would something based on the following
work (after suitable extension to cover all the potential
problems I'm missing, like avoiding corrupting the user's FP

  a)  upon entry into any device routines that use FP, you first
      increment global_using_fpregs.
  b)  upon exit from any device routines that use FP, you
      decrement global_using_fpregs (and panic if it is already
  c)  in the main interrupt handler, if global_using_fpregs > 0,
      save the FP registers, otherwise you don't have to.

  Or is a scheme like this what you meant by ``very careful about
saves and restores''?  :)

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