How is hyperthreading handled on amd64?

[Pete French]

> After building a kernel with SMP, I do see 2 logical processors,
> and top(1) shows both of them doing their thing.  However,
> machdep.hypterthreading_allowed is set to 0.  Is the amd64 kernel
> ignoring this value, or is top(1) reporting incorrectly?

It seems to be a bug - if you have an SMP kernel you cant turn off
hyperthreading, no matter which knobs you twiddle. Not amd64 specific
either. Even with all the hyperthreading variables off and with the
masks showing that processes should not be scheduled on the second CPU's
I still see stuff on 0, 1, 2 and 3.

-pcf.