[PATCH] mm: Add infrastructure for basic memory

Stephen Tether tether at slac.stanford.edu
Wed Feb 20 22:24:05 UTC 2013

On 02/20/2013 06:11 AM, rtems-devel-request at rtems.org wrote:
> Today's Topics:
>    1. Re: [PATCH] mm: Add infrastructure for basic memory
>       management (Sebastian Huber)
>    2. Re: RFC: ARM GCC changes (Sebastian Huber)
>    3. [PATCH] libblock: Do resource allocation in one place
>       (Sebastian Huber)
>    4. Re: [PATCH] mm: Add infrastructure for basic memory
>       management (Gedare Bloom)

I, too, would prefer integer encodings of the memory attributes rather
than having a setter function for each attribute, since it would make it
possible to change multiple attributes at once.

Will these proposed MM routines perform the cache cleaning that is
required for some types of attribute changes? I have in mind the loading
of code into memory that is writable and non-executable and then
changing the memory to read-only executable.

Have you considered exporting memory types that reflect the memory's
function rather than (or in addition to) the raw MMU attributes? The
trouble with the latter approach is that it allows arbitrary
combinations of attributes many of which may be useless or even harmful.
Here's a set of functional attributes that seem good to me, at least for

Function                ARM MMU settings
---------------         ------------------------
CODE                    cached
DATA                    non-executable, writable, cached
CONSTANTS               non-executable, cached
CALL_STACK              non-executable, writable, cached
DMA_BUFFER              non-executable, writable
PAGE_TABLE              non-executable, writable, cached
DEVICE                  non-executable, ordered or device

Independently of these functional settings one could have a SHARED flag
and a cache-policy setting, e.g., WRITE_BACK or WRITE_THROUGH. Add a
number of customizable functional settings to cover corner cases.

Even this scheme isn't completely orthogonal. On the ARM, for example,
SHARED|WRITE_THROUGH|PAGE_TABLE would be invalid since shared page
tables must be WRITE_BACK. Then, too, perhaps this sort of thing doesn't
belong in the lowest level of MMU support.

- Steve Tether
  SLAC DAT Group

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