Re: [Xenomai] ipipe/x86: do not restore during context switch

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

On 02/06/2013 06:33 PM, Jan Kiszka wrote:

> On 2013-02-06 18:09, Gilles Chanteperdrix wrote:
>> On 02/06/2013 06:03 PM, Jan Kiszka wrote:
>>
>>> Gilles,
>>>
>>> do you remember if this core-3.4 change was a performance optimization
>>> or a necessary fix? Also, I'm not yet understanding why we need all the
>>> #ifdefs except for the first one which forces fpu.preload to 0.
>>
>>
>> It is a performance optimization, without it, we systematically hit the
>> maximum latency when the timer would tick during a context switch which
>> restores the FPU. Note that if you change that, you will probably break
>> -forge.
> 
> According to the Intel folks who introduced eagerfpu, xsave, or at least
> xsaveopt (which I didn't implemented yet) is now faster than serializing
> clts/stts. On the other hand, the worst case is a full SSE + AVX restore
> while the target RT task is not depending on the FPU.


Without xsave, we never restore fpu if the RT task never used it. This
changes with xsave?

-- 
                                                                Gilles.