From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <454B1363.8080805@domain.hid> Date: Fri, 03 Nov 2006 11:01:07 +0100 From: Gilles Chanteperdrix MIME-Version: 1.0 Subject: Re: [Fwd: Re: [Xenomai-help] invalid use of FPU in Xenomai context] References: <4548C170.1000504@domain.hid> <4548F8BE.90809@domain.hid> <454901B6.80606@domain.hid> <454B0FA0.9080307@domain.hid> In-Reply-To: <454B0FA0.9080307@domain.hid> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit List-Id: Help regarding installation and common use of Xenomai List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: Xenomai help 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 - 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. -- Gilles Chanteperdrix