How is hyperthreading handled on amd64?

[Pete French]

> I don't see this behaviour on i386 - my desktop, for example uses only
> CPU 0 if the sysctl is 0, and switches between 0 and 1 when the sysctl
> is 1.

Uh, a typo on my part - substitute *machine* for *kernel*. Sorry!

A machine with one physical processor works fine - ccan enable and disable
hyperthreading as pper the manual. A machine with two physical processors
always gets hyperthreading enabled though. All of these machines are
running 6.0 currently - though I will verify that the bug is still there
on 6.1 when I get a moment to do an update on them.

-pete.