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Date: Tue, 19 Jun 2012 11:38:08 +0200
From: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
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Subject: Re: [Xenomai] Sharing memory between rt-posix and standard-posix
 apps
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To: Wolfgang Grandegger <wg@grandegger.com>
Cc: Xenomai@xenomai.org

On 06/19/2012 11:29 AM, Wolfgang Grandegger wrote:
> On 06/19/2012 11:04 AM, Gilles Chanteperdrix wrote:
>> On 06/19/2012 10:52 AM, Wolfgang Grandegger wrote:
>>> On 06/19/2012 10:50 AM, Gilles Chanteperdrix wrote:
>>>> On 06/19/2012 10:49 AM, Wolfgang Grandegger wrote:
>>>>> On 06/19/2012 10:33 AM, Gilles Chanteperdrix wrote:
>>>>>> On 06/19/2012 10:25 AM, Wolfgang Grandegger wrote:
>>>>>>> Hello,
>>>>>>>
>>>>>>> the shared memory support of Xenomai's POSIX skin currently only support
>>>>>>> shared memory between rt processes.
>>>>>>
>>>>>> These services are for sharing memory between user-space and
>>>>>> kernel-space. In order to share memory between user-space processes, you
>>>>>> can use plain linux shared memories.
>>>>>
>>>>> But then I will get mode switches due to page faults, right?
>>>>
>>>> No, there should not be any page fault.
>>>
>>> But that's what we observe on a i.MX53 and i.MX6Q board. IIRC, the first
>>> access usually results in a page fault.
>>
>> And you do not get the same behaviour with posix skin shared memories ?
> 
> Right, and that's because do_pagefault() is called somewhere in the
> initialization, IIRC. But I will redo my tests now.

Yes, I forgot about that, we pre-fault shared memory on ARM, in order to
avoid the page fault on first access.

> 
> Anyway, if I use the POSIX skin the shared memory functions in there
> will be used automatically. But I could use  __real_shm_open and
> __real_mmap, right?

Or you can used the wrapped version, but without enabling support for
xenomai posix shared memories in the kernel configuration.


 ... and access the page once per page in the
> initialization phase. Then no more page faults should happen.

Yes. It is a bit ugly, but should work.

-- 
					    Gilles.