XMM register usage in the kernel

[Matthew Fleming]

> The KPI consists of two functions, fpu_kern_enter() and
fpu_kern_leave().
> The pair should brace the region of code that intend to use
coprocessor.
> Caller shall provide the context save area, I expect that usually this
> memory will be allocated as part of larger subsystem data structure.
> 
> fpu_kern_enter() allows the nesting, assuming fpu_kern_leave() is
called
> proper number of times to pop the context stack.

Isilon would like it if something like this functionality ended up in
CURRENT.  We have some hacks that do something similar.  We use SSE2
instructions to significantly speed up computing ... stuff. :-)

Thanks,
matthew