Re: [ECOS] Re: Regd. the MMU in eCos1.3.1

[Jonathan Larmour <jifl at eCosCentric dot com>]

Rajan Srivastava wrote:
> Hi,
>
> The query launched by Mr Prabal is very much valid.
> In follow-ups, it is said that why at all memory locking 
> is needed, let me elaborate the need.
> Suppose I want some data to be protected against
> accidental writes. If the page/segment is write protected,
> eCos could generate an exception (write attempt on
> RO page/segment) when some thread tries to
> write on this area due to some coding bug. 
> So testing will become very easy.
>
> Kindly see that eCos is getting so popular that
> it has been widely accepted for realtime 
> embedded softwares, so these kind of lock
> requirements should be met.

I agree, but see below....

> Or you can guide us, how to write memory locking for
> a given processor, eCos too can accept 
> this facility in its overall source-code

eCos is lightweight, and the burden of levels of page tables and so on 
would make eCos less suitable for the low-end, so at best it would be an 
option.

Some people say that we should always protect the text segment, and while 
that's _mostly_ true, it isn't always, not least for things like software 
breakpoints, nevermind self-modifying code (anathema in general, but can 
be a useful tool in resource-constrained systems).

It would be good to have some scalable, flexible, lightweight and generic 
API to allow people to fiddle with the MMU mappings, but this is _hard_. 
MMUs are one of the areas that differ radically between MMUs and trying to 
design an API in such a way as to prevent the API making life difficult 
for the HAL porter would be tricky. So it hasn't happened.

Of course anyone is welcome to add HAL-specific MMU fiddling macros for 
individual targets/variants/architectures, but something requiring the 
amount of analysis that a generic API would require, would need funding 
before it happens.

Jifl
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