Subject: Re: kernel mapping

From: Dan Malek <[email-masked]>
Date: Tue, 16 Jan 2001 11:50:15 -0500

Ralph Blach wrote:
>
> Why do we need simulated bat registers.

To improve performance.  Right now, on the 4xx there is the
concept of "pinned" TLB entries to reduce/eliminate TLB misses
on large mapped areas (like kernel text/data or I/O).  The 8xx
does this in some custom applications as well.  These are just
hacks that are headed down a disastrous maintenance path that
need to be stopped now for a more generic solution.

I have been experimenting with many different methods of using
the "large" page table sizes through the generic memory management
methods that already exist in the kernel.  I believe I can wrap
the concept of the pinned TLB entries into the same logic as BAT
register management on the bigger processors.  Hence, I call them
simulated BAT registers....the semantics aren't quite the same.

The BAT registers are a really good thing, and although the large
page size TLB entries are more flexible, they require more software
overhead.  I would like to make some generic Linux MM modifications
to help us support variable page sizes, but I suspect that will
never happen.

	-- Dan

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