> The "cpuid" instruction is not available on all x86 CPUs and it is not
> easy to use.

It's certainly difficult to use, yes. But being unavailable on old cpu's
is not a problem, since it _is_ present on any chip that has mmx. You can
just preset the output bits and catch any generated traps. But I guess you
knew all of that.

> I would prefer something like a sysctl(3) variable.

That would be nice, if quite a bit less portable.

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