Re: Using kernel_fpu_begin() in device driver - feasible or not?

[Steffen Persvold <sp@scali.com>]

Alan Cox wrote:
> 
> > 1. Would it be sufficient to just bracket all fpu-using code code by
> > kernel_fpu_begin()/kernel_fpu_end()?  If not, what problems could I
> > run into?
> 
> You can do that providing you dont
> 
> > 2. Would it be OK to go to sleep inside such a region, or should I
> > take care to avoid that?
> 
> You can't sleep in such a region - there is nowhere left to store the
> FPU context
> 
> > 3. Perhaps I should call init_fpu() at some point as well?  If so,
> > should it be done before or after kernel_fpu_begin()?
> 
> After
> 
> > 4. Is there any difference between doing this in the context of a user
> > process (implementation of an ioctl) compared to doing it in a
> > daemonized kernel thread (created by a loadable kernel module)?
> 
> The kernel thread is actually easier, you can happily corrupt its user
> FPU context by sleeping since you are the only FPU user for the thread.
> Not nice, not portable but should work fine on x86 without any of the
> above for the moment.
> 
> You should probably also test the FPU is present and handle it accordingly
> with polite messages not an oops 8)

So are kernel_fpu_begin()/kernel_fpu_end() (and also init_fpu()) necessary in a kernel thread at all
? Does kernel_fpu_begin()/kernel_fpu_end() take care of SSE and MMX registers too (I'm aware of the
extra "emms" needed in the MMX case) ?

Regards,
-- 
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