WARNING OFF TOPIC SLIGHTLY: 8xx MMU w/8MB pages

[Dan Malek]

Christer Weinigel wrote:

> ........  Set the 4/16k bit in
> MD_RPN to 1 and it'll work.  Apparently, the two page size bits in
> MD_TWC and the bit från MD_RPN together form the real page size.  The
> documentation is a bit misleading here.

That is the proper thing to do, but not exactly the reason.  You
should also set MD_CTR.TWAM = 1, since the problem you are solving
is telling the MMU not to use 4K/1K subpages.


> Now I'd just like to know why I can't get 8 MB pages working with the
> Linux page tables.


How much software are you trying to modify in the generic Linux MM
functions?  If none, it isn't likely to work except in one condition.
You will have to wire the pages into permanent TLB entries, and
move the kernel VM map around to ensure dynamically allocated
pages don't overlap the 8M "pages".  This is the first and easiest
to do, but......


> ....  The performance boost from putting the Linux
> kernel in an 8 MB page on a MPC850 is rather impressive, most lmbench
> system call benchmarks went twice as fast.


....lmbench is not necessary representative of embedded applications.
Performing the above modification to some of the embedded applications
I have developed would reduce the performance because the number of
TLB entries in the 850 or 823 is rather small.

As I have said in previous messages, you can find quick little
hacks like this that may help a specific application, but they
are not generically beneficial.  The real solution is to significantly
modify the Linux VM macros and functions to allow a better formatted
page table and reduce the amount of instructions in the TLB handler.
Further, we could leverage a variety of page sizes simultaneously.
This has been on my list for a long time, but isn't a trivial
weekend project (especially with the VM changes in 2.3.xx).


	-- Dan

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