[PATCH 0/3] 8xx: Large page(8MB) support for 2.4

[Joakim Tjernlund]

Dan Malek <ppc6dev at digitaldans.com> wrote on 2011/10/10 20:03:53:
>
>
> On Oct 10, 2011, at 9:45 AM, Joakim Tjernlund wrote:
>
> > That is an easy port but I will have to do that blind. Would you
> > mind take this for a spin on 2.4 first?
>
> My current system is running 2.6, so I don't have much
> interested in testing 2.4

Too bad as I won't be able to run 2.6 at all.
However, I just sent you a dry port to 3.0 of the large page stuff to
you. Happy testing.

>
> > The more interesting part is if one should use other sized(16K or
> > 512K) large pages too?
>
> My thought long ago was most of the 8xx systems have rather small
> real memories, so the larger pages, especially 512K may be too wasteful.
> I've always been a fan of keeping the TLB handlers tiny and simple,
> rather then spending the instructions doing complex replacements.
> Remember, this also affects the I- and D-cache, so a more frequent
> and trivial PTE update could very well gain larger system performance
> than the management of larger pages with more complex code.
> With all of the bug fix code in the handlers, maybe a larger page would
> be better.
>
> > Those should be useful for user space but it is a lot of work. I
> > haven't checked
> > what large page support for user space is in 2.6 for ppc though.
>
> The 2.6/3.0 kernel supports different, but fixed, page sizes.  IIRC,
> anything
> over 64K may require distribution rebuilding to realign code/data
> sections
> to more restrictive boundaries.  Maybe a 16K page would show some
> benefit.
> I'll try to make some time to play with it.

Unfortunately 3.0 does not have any free PTE bits now. I had to move
_PAGE_SPECIAL to the last available one as the new _PAGE_PSE needed its place.
Don't know what _PAGE_SPECIAL is but if it cannot be removed one can always skip
_PAGE_WRITETHRU again.

 Jocke