Re: [patch 02/10] powerpc: Move CONFIG_PPC64 into CPU selection

Segher Boessenkool writes:

It would be good to converge the 32- and 64-bit implementations,
and/or allow a single kernel image to work on both 32-bit and
64-bit systems,  The current situation is far removed from that,
so this patch is a bit premature in that aspect.

There will always be differences in emphasis and focus between 32-bit
and 64-bit.  For example, the vast majority of 32-bit systems are UP,
with a few 2-way SMP systems, an insignificant number of 4-way
systems, and nothing bigger than 4-way.  In contrast, most 64-bit
machines are SMP, with enough large SMP machines (64-way and bigger)
to be worth worrying about.  Hence the 64-bit code focuses on SMP
scalability whereas the 32-bit code focuses on performance for UP and
2-way systems - which is why the hash table invalidations get batched
up on 64-bit machines but not on 32-bit machines, for instance.
Similarly, 64-bit supports large memory (>4GB) and NUMA machines,
whereas 32-bit doesn't.

Paul.