Re: Questions about copy-on-write

[Marcus Brinkmann]

At Wed, 27 Oct 2004 21:33:09 +0100,
Sam Mason <address@hidden> wrote:
> Neal H. Walfield wrote:
> >physmem holds mappings
> >between virtual frame (per-task handles for physical memory blocks)
> >and physical frames.
> 
> Is this where most of the memory protections stem from?  Tasks can't
> ask for arbitrary pages of memory but they can ask physmem for a new
> (otherwise unused) page of memory or they can ask for a specific page
> of memory out of a container.

Right.
 
> If that's true then I think my confusion was coming from a belief that
> this mechanism was in L4 iself, rather then physmem.

L4 only manages the page tables, and provides the first task in the
system with all of the physical memory in a 1:1 mapping (vaddr == paddr).
 
> I think the next challenge for me is to figure out how a task can
> revoke a mapping it gave to another task.  I guess I'll have to give
> the L4 a bit of a read. . .

L4_Unmap Systemcall (see the specification).  Note that L4 supports
recursive mapping models.  We don't really make use of that, though.

Thanks,
Marcus