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Re: processes vs threads (MMU protection)


Armando Visconti <armando.visconti@st.com> writes:

> I would like to ask to the mantainers why the work
> done by 3G lab on 'memory protection' has not
> been included in eCos v2.0.
> 
> Is not stable enough?

That work was done a long time ago, against a very old version of
eCos. It only addressed the issues that 3G Labs were concerned about
at the time. It was also very specific to the ARM architecture.

> Don't you think it would be great for a MMU
> based system to get benefit from a memory
> protected OS ?
> Any plan in this direction?
> 

Thomas Binder has recently submitted a patch to implement a form of
process-like memory protection for PowerPC based devices. While I have
not looked at this in great detail yet, it appears to suffer from some
of the same problems as the 3G Labs work: it may be too specific to
the PowerPC MMU model.

More generally, eCos was not designed to be a memory-protected
operating system. I have implemented such systems in the past, and the
approach taken needs to be very different. When originally
implementing eCos we explicitly decided that a process-level memory
protection system was not something we wanted to allow
for. Consequently things like the APIs, kernel data structures and
algorithms are not suitable.

To add a fully general, portable, process model to eCos would be
months, if not years, of work. It would require significant rewrites
of the kernel, the HALs and many other packages. The consequent
destabilization of the source tree would be a significant blow to
eCos' usability. 

My general attitude is that if you want processes, use an operating
system that already supports them, like Linux.

-- 
Nick Garnett                    eCos Kernel Architect
http://www.ecoscentric.com      The eCos and RedBoot experts


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