Re: [Xenomai-core] [RFC] FPU support

[Philippe Gerum <rpm@xenomai.org>]

Gilles Chanteperdrix wrote:
> Paul wrote:
>> Hi Giles
>>
>> On Tuesday 27 January 2009, Gilles Chanteperdrix wrote:
>>> So, the question is: are there people around who either:
>>> - need FPU support for kernel-space real-time threads;
>>> - do not want to pay the price of a trap when using the FPU in user-space.
>> One application that I work on uses floating point math in kernel space, but 
>> it is a constant source of problems - Being forced to migrate to user space 
>> would be a welcome move.. The only question is how much of an overhead does 
>> the trap incur ?
> 
> The answer probably depends on the machine. You can probably estimate it
> for x86 by using rdtsc in well chosen places. What you need to measure
> is the time between the fpu fault in user-space and the beginning of the
> fpu fault handler in kernel-space, then the time between the end of the
> fpu fault handler and the return to user-space.
> 

Enabling the I-pipe tracer would help in determining how many cycles are burnt
from early trap handling back to the supervisor exit point; if you do this, you
may want to keep IPIPE_DEBUG_CONTEXT off, since this adds some overhead on low
end hw. user/supervisor transition time would have to be added as well, but this
is likely not the most significant portion of the overhead.

-- 
Philippe.