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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

    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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