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From: Roger Larsson <roger.larsson@norran.net>
To: linuxppc-embedded@ozlabs.org
Date: Sat, 14 May 2005 04:55:29 +0200
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Subject: floating point in kernel (Re: insmod problem)
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On Friday 13 May 2005 15.46, Kumar Gala wrote:
> On May 13, 2005, at 8:14 AM, Mark Chambers wrote:
> >  > You're using floating arithmetic in the kernel.
> >  >
> >  > That's not supported.
> >  >
> >  > Regards
> >  >
> >  > Pantelis
> >
> > Just for my edification:=A0 Is floating point itself not supported
> > (floating
> >  point register
> >  corruption or something like that) or just linking to user space
> > libraries?
>
> Use of floating point registers and instructions.  Its possible to use
> FP in rare cases inside the kernel buts its extremely discouraged.
>

The kernel is doing lazy saving of FP registers, only saving for user=20
processes that uses them.  And especially avoiding to save for every
interrupt!

So if you like to use any FP (registers) in kernel you have to:
=2D disable kernel preemption (YES)
=2D save previous value
=2D do your stuff
=2D restore
=2D enable preemption

I remember a preemption bug caused by using MMX/SSE to copy
memory uncached.

/RogerL