Re: 2.6.6-mm5

[ebiederm@xmission.com (Eric W. Biederman)]

Andi Kleen <ak@muc.de> writes:

> ebiederm@xmission.com (Eric W. Biederman) writes:
> 
> > Currently I know of a safe version that will work on x86 on processors
> > with sse support.   And I how to generate 64bit I/O cycles with using
> > mmx or x87 registers,  but don't know if I can write code that touches
> > the FPU registers that is interrupt safe.
> 
> As long as you save/restore cr0 and the FPU registers and do clts
> interrupts are not a problem.  In fact interrupts are even easier that
> process context, where you need preempt_disable().

The saving and restoring is where things are looking icky.

It does not look like that kernel_fpu_begin() will work safely in
an interrupt context.

The generic x86 variant is to do:

fild
fistp

Which works for 64bit values because the floating point registers
have a 64bit mantissa.

I suppose I could unconditionally save the x87 floating point registers
to a local variable, but that sounds like a terribly expensive operation.
At least with kernel_fpu_begin() the floating point save only
needs to happen once, per context switch.

With SSE it is easy to save just a single register.  For the x87 I don't
see how to do that.  Beyond the stack based nature of the x87 there is
the question if the registers are in mmx mode or not.

I guess a conservative always correct version would be a place to start.

Eric