L4 for asm-free OS'es?

William A. Gatliff bgat at billgatliff.com
Mon Apr 8 18:04:59 CEST 2002


Wow!  Thanks for all the feedback, guys!  :^)

I obviously need to tighten up my terminology a bit, and spend even
more time in some L4 Kernel sources... (I have Hazelnut here now).

On Mon, Apr 08, 2002 at 05:41:50PM +0200, Robert Kaiser wrote:
> If by "work the MMU" you mean to setup/manipulate page tables etc.,
> then no, neither sigma0 nor any other entity outside the µkernel
> knows how to work the MMU. The Kernel provides a platform independent
> mechanism (IPC) for pages to migrate between different processes. So, if a
> process owns a set of pages, it can map or grant (some of) these pages to
> another process. The reciever of the pages can map (some of) these pages
> on to another process, and so on.

Yea, that's what I meant by "work the MMU".  But you gave me the
answer I was looking for--- that the Kernel owns the MMU, not user
processes, *and* there's enough of a consistent API there that an OS
running under an L4 Kernel need not concern itself with the MMU.

> After what's been said above I think it is probably clear now that
> this is already how things work with L4. However it is not sigma0
> that does the mapping, but the kernel. Note that L4 strictly
> seperates between policy and mechanism: the kernel provides only the
> mechanism (i.e. how to "work the MMU"), while all the policy
> (e.g. page replacment algorithms, etc.) is to be implemented in user
> space.

Right.  This is what I'm after.

> There are e few things besides the syscall interface that are (and
> will always remain) platform dependent, though they need not
> necessarily be implemented in assembly language.

Sure.  But it looks like the number of them gets smaller by going with
L4.  :^)

> It all sounds so familiar ... Apparently I have interestst very
> similar to yours :-). My main problem is time to actually _do_ all
> these nice things

It's different when you make a living at it, of course, but not too
different.  I don't yet have the time, either!  :^(


b.g.
-- 
Bill Gatliff
bgat at billgatliff.com




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