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SubjectRe: RFC: [2.6 patch] better i386 CPU selection
Mikael Pettersson wrote:
> There is to the best of my knowledge nothing in 2.6.0-test4
> that prevents a kernel compiled for CPU type N from working
> with CPU types >N. Just to prove it, I built a CONFIG_M486
> 2.6.0-test4 and booted it w/o problems on P4, PIII, and K6-III.

You may be right, although I wonder if there are real problems like an
SMP Pentium kernel not setting up MTRRs when run on an SMP P3.

The main problems are:

1. Optimisation. A kernel optimised for P3 but compatible
with 486 needs to use 64 byte cache line alignment, and TSC
for timing, but not use any SSE instructions.

This is different from a kernel optimised for a 486.

2. The CPU types are not a total order. Say I want a kernel
that supports Athlons and a Centaur for my cluster. What
CPU setting should I use? What CPU setting will give my the best
performing kernel - and is that the same as the one for smallest
kernel?

3. Like 2, but for embedded systems. I'm (hypothetically)
selling a cable modem which was originally based on one
CPU, but we changed to a different one because it was
cheaper. I need to send out a firmware upgrade, and it is
convenient to use a kernel which can run on either model.
But I don't want to compile in support for every x86,
because space is tight, and I want it to run as fast as it
can given that it could run on either of the two chips.

> How are 1 and 2 different? Both need support for CPU type N
> or higher. Since a kernel configured for a lower CPU type
> still works on higher CPU types, where is the problem?
> (In case 2 configure for PIII and use that on PIII and P4.)

I'm not sure if an Athlon is "lower" than a PII or not.... Which do I
option do I pick, to run on either of those without including
redundant stuff for older CPUs?
-- Jamie
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