Re: [Fwd: Re: [Xenomai-help] invalid use of FPU in Xenomai context]

[Jan Kiszka <jan.kiszka@domain.hid>]

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Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>> Jeff Webb wrote:
>>
>>> Jan Kiszka wrote:
>>>
>>>> Jeff Webb wrote:
>>>>
>>>>> Does anyone else have an AMD system that can verify my results?
>>>>
>>>> I have an old Athlon 800. Maybe we are lucky and it exposes the problem
>>>> when the kernel is optimised for it. I'm going to give this a try, but
>>>> it may take a few days (and a free time slot).
>>>
>>> Thank you.  I appreciate you giving it a try when you get some free
>>> time.  I was able to work around the problem by writing the queue data
>>> in smaller chunks (or use an i686 kernel), so I am not in urgent need of
>>> an immediate fix.  I do think it's important to fix this bug eventually,
>>> so I didn't want it to slip through the cracks.
>>>
>>>
>>>>> The problem seems to be connected with the size of writes to Xenomai
>>>>> pipes.  This example uses POSIX message queues, but I had a similar
>>>>> problem a while back with RTAI pipes.  Maybe this tells us the problem
>>>>> is in the nucleus pipe code?  Just a guess.  The problem seems to
>>>>> affect
>>>>> both 2.4 and 2.6 systems, and goes back to at least Xenomai 2.2.1.
>>>>
>>>> Maybe, maybe not. Pipes remain fairly unrelated to FPU usage, so there
>>>> is still /at least/ one piece missing in the puzzle.
>>>
>>> True.  It is very strange that the amount of data in the write call ends
>>> up affecting the FPU context.
>>
>>
>> I found the reason: "3-dimensional" memcpy (__memcpy3d/_mmx_memcpy)
>>
>> http://lxr.free-electrons.com/source/include/asm-i386/string.h#285
>>
>> It's an optimised memcpy for 3DNow CPUs that is used with blocks >= 512
>> bytes. It messes up with the FPU state and may get trapped by other
>> issues as well (blind access to "current" in order to test
>> in_interrupt()). I don't have an answer for this right now beyond "don't
>> switch on AMD optimisations when using Xenomai". But that's a bit
>> unsatisfying.
>>
>> Another way would be to wrap any memcpy access from Xenomai context, but
>> that's likely impractical (think of all the drivers).
> 
> I see other ways to solve this issue:
> - either we disable the use of the mmx memcpy in string.h if
> ipipe_current_domain is not root

This is what came to my mind as well meanwhile.

> - or we allow the exception to happen for threads in primary mode with
> the XNFPU bit set and call xnarch_restore_fpu in xnpod_fault_handler in
> this case.

Given that this is a special case for a subset of x86[_64] CPUs, I
rather think we should go for the first variant. Should be simpler.

Jan


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