linux-kernel - Re: [PATCH -mm] Revoke core code: fix nommu arch compiling error bug

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Date:	Tue, 27 Mar 2007 11:05:13 +0100
From:	David Howells <dhowells@...hat.com>
To:	"Mike Frysinger" <vapier.adi@...il.com>
Cc:	"Pekka J Enberg" <penberg@...helsinki.fi>,
	"Andrew Morton" <akpm@...ux-foundation.org>, bryan.wu@...log.com,
	linux-kernel@...r.kernel.org, "Hugh Dickins" <hugh@...itas.com>,
	"Alan Cox" <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH -mm] Revoke core code: fix nommu arch compiling error bug 

Mike Frysinger <vapier.adi@...il.com> wrote:

> > [*] The FRV, for example, does have some limited protection capability - but
> > it is really limited and not really useful in this case.
> 
> how so ?

There are a limited set of protection registers (At least 8 insn and 8 data)
that can permit tiles of protection that are a power-of-2 megabytes in size.
However, these also have to be used to grant the kernel access to h/w
(including RAM).

Note that it's not possible to shift windows around in response to faults
because fault reporting is asynchronous - the entire remaining instruction
queue will be executed *before* the exception is actually raised to the
kernel.

> the Blackfin processor lacks a MMU but it does have a MPU (memory protection
> unit) which allows granularity down to 1k page sizes ... so for future
> releases, we plan on integrating optional support for this so that you could
> have processes protected from each other and the kernel protected from all
> the processes ... so in our case, we might actually be able to support
> revoking of maps because we would have that region of memory ear marked as
> unaccessible ...

That sounds reasonable.  However, I suspect that most NOMMU CPUs won't be able
to do that.  In effect you're creating a third option, I think (MMU, NOMMU,
MPU).

> note that the Blackfin processor manuals confusingly call this aspect
> of the chip an "MMU" ... dont be fooled !

Same for FRV.  It is memory management - it just doesn't include virtual
memory mapping.

David
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