Memory protection in RTEMS

[Gedare Bloom]

Hi Mohammed,

Hardware support for memory protection is not uniform across processor
architectures. Generally speaking, memory protection is
non-deterministic, although some restrictions may be imposed to reduce
the non-determinism by limiting the number of protected memory
regions. Changing the protection domain when task-switching also
greatly increases the system-level overhead for context switching,
thus causing difficulty to WCET estimates and schedulability analysis.
Furthermore, memory protection when enabled increases the average
memory access time. Most real-time systems in the past have been
"single-user, single-program, narrow networking" which precludes any
real need for protection. Now we see multi-program, multi-user, and
wide networking interfaces, and the need for memory protection is
becoming more of an issue.

Gedare

On Thu, Apr 9, 2015 at 12:12 AM, Mohammed Saeed Khoory
<Mohammed.Khoory at eiast.ae> wrote:
> Hi,
>
> I find this topic interesting as well. I've had a general question about memory protection that I was hoping someone knowledgeable could shed light on.
>
> I've noticed that some RTOS's (not just RTEMS) don't implement memory protection and prefer to stay that way. I think real-time Linux also explicitly disables memory protection by having real-time tasks run in kernel mode. Is there something about memory protection that makes it disadvantageous to real-time computing in general? Does it introduce non-determinism? Or is it in order to reduce complexity, and maybe to keep the address space flat and simple?
>
> Thanks!
>
>> -----Original Message-----
>> From: users [mailto:users-bounces at rtems.org] On Behalf Of Gedare Bloom
>> Sent: Thursday, April 02, 2015 7:39 PM
>> To: Tomasz Gregorek
>> Cc: users at rtems.org
>> Subject: Re: Memory protection in RTEMS
>>
>> On Thu, Apr 2, 2015 at 11:19 AM, Tomasz Gregorek
>> <tomasz.gregorek at gmail.com> wrote:
>> > Hi Gedare
>> >
>> > Thank you for your comments, they answer my question.
>> >
>> > We will look at other RTOSes this time.
>> >
>> OK, I'd be happy to hear what you find or settle on. It's a topic that I
>> have continued interest in working on for RTEMS.
>>
>> Gedare
>>
>> > Thanks
>> > Tomasz
>> >
>> > 2015-04-02 17:01 GMT+02:00 Gedare Bloom <gedare at rtems.org>:
>> >>
>> >> Hi Tomasz,
>> >>
>> >> It is possible to implement some memory protection between threads.
>> >> As you noticed, RTEMS is a SASOS with "one process" when viewed from
>> >> the POSIX interface. Generalizing memory protection in such a system
>> >> is hard. The most common approach I know of is to provide separate
>> >> protection domains for each thread stack. In theory, you could create
>> >> a protection domain for the RTEMS "kernel", but I'm not aware of
>> >> anyone that has tried hard to do that. You may also look for the
>> >> "Zero Kernel" work some folks did as a research project at Univ of
>> >> Idaho I think it was.
>> >>
>> >> If you want memory protection, you'd have to tailor it to your
>> >> specific application requirements and set-up the MMU/MPU to do it.
>> >> Final note: Since there is no notion of privilege in RTEMS, any
>> >> thread can always disable or modify the protection domains since
>> >> priviliged registers and instructions are available. Thus, while you
>> >> may add memory protection, it would not be for a complete security
>> solution.
>> >>
>> >> Gedare
>> >>
>> >> On Thu, Apr 2, 2015 at 10:04 AM, Tomasz Gregorek
>> >> <tomasz.gregorek at gmail.com> wrote:
>> >> > Hi
>> >> >
>> >> > We are investigating possible OSes for a new product where one of
>> >> > the requirements is memory protection between processes.
>> >> >
>> >> > Does RTEMS supports memory protection? If not than is it planned to
>> >> > implement memory protection?
>> >> >
>> >> > This is what I found so far:
>> >> >
>> >> > RTEMS is single process, multiple threads and as such have
>> >> > everything in single memory space.
>> >> > http://comments.gmane.org/gmane.os.rtems.user/19858
>> >> >
>> >> > Gedare's blog with his implementation of memory protection from
>> >> > 2011/2012.
>> >> > http://gedare-csphd.blogspot.be/2011/12/rtems-memory-protection-api
>> >> > .html but it seems that this code didn't get to the main.
>> >> >
>> >> > Project "MMU Support" by Aanjhan Ranganathan from 6 years ago:
>> >> > https://devel.rtems.org/wiki/Projects/MMU_Support
>> >> >
>> >> > Source code configures MMUs but it seems it do it to enable cache
>> >> > only, not the memory protection. Though I did only simple search
>> >> > through the code.
>> >> > There are exception handlers for some architectures/CPUs but only
>> >> > for few if not for only one.
>> >> >
>> >> > Best regards
>> >> > Tomasz Gregorek
>> >> >
>> >> > _______________________________________________
>> >> > users mailing list
>> >> > users at rtems.org
>> >> > http://lists.rtems.org/mailman/listinfo/users
>> >
>> >
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