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    SubjectRe: PPro for a PII in make config
    > Those options mostly just affect optimization.  (The only
    > compatibility issue is that kernels not compiled for "386" will use
    > instructions added in the '486 and will not run on 386s and old
    > clones.) Selecting "PPro" would probably give you the best
    > performance.

    Below is part of an e-mail from Michael Chastain, with his analysis of why
    there are probably problems with '486s as well. The existing
    recommends CONFIG_M586 for 5x86s, and I've seen a case where someone
    followed its advice. I've prepared a patch which changes this
    recommendation and updates the help text to reflect newer CPUs and
    non-Intel CPUs. The patch is at
    Trevor Johnson

    > In rare cases, it can make sense to specify "Pentium" even if
    > running on a 486: the kernel will be smaller but slower.

    I think this information is old.

    See include/net/profile.h; CONFIG_NET_PROFILE enables this file.
    net_profile_stamp has different implementations depending on the
    CPU model. I don't have an opcode chart available but I will bet
    that the 586/686 implementation of net_profile_stamp is issuing
    an instruction not implemented on the 486.

    > From my cursory reading of arch/i386/kernel/head.S, it looks to me like
    > the 4 MB pages are enabled when __SMP__ is defined. A kernel compiled
    > with CONFIG_M586 and SMP might not run on a '486, then.

    That section of code gets executed for non-primary CPU's. So yes,
    I think a kernel compiled with SMP and CONFIG_M586 is likely to have
    a problem there. (There are other problems trying to run SMP on
    a 386 or a 486).

    > Also, wouldn't it be compiled with the the space-wasting 16-byte
    > alignments from -m486? There's a "tsc" flag in /proc/cpuinfo, so I'd
    > assume the RDTSC feature is used only when it's detected.

    You are correct. It is a run-time flag, not a compile-time flag.
    The code is in arch/i386/kernel/time.c, function time_init. It chooses
    between fast code and slow code based on x86_capability.

    Same with the 4-megabyte pages; they also get initialized based on a
    run-time test of x86_capability.

    > Kernels compiled with CONFIG_M386 are smaller and slower; perhaps there
    > was a mistake in the original text. Thanks for asking about this.

    I bet that kernels compiled with CONFIG_M386 are larger than kernels
    compiled with CONFIG_M586, because CONFIG_M386 compiles a bunch more
    code for verify_area calls. It would be worth checking that out.

    So to recapitulate: I don't like saying that a kernel built with
    CONFIG_M586 will run on a 486 machine. We have looked at four cases
    where this might fail:

    #1 4-megabyte pages
    #2 do_fast_gettimeofday
    #3 include/net/profile.h
    #4 4-megabyte page table SMP code in arch/i386/kernel/head.S.

    #1 and #2 are dynamic code based on x86_capability so there is no
    problem. #3 is a definite gotcha. #4 may be a gotcha.

    Based on this, I recommend dropping this line and falling back to
    a simpler position: kernels built for a less advanced processor
    will run on a more advanced processor, but not vice versa.

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