How is hyperthreading handled on amd64?

[Pete French]

> Again, not what I'm seeing here.  It appears as if the machine behaves
> identically no matter what machdep.hyperthreading_allowed is set to.
> The machine I'm testing on only has a single physical processor.
>
> I've been running ubench multiple times to test this, and I see processes
> on CPUs 1 and 0 no matter what I set the above sysctl to.  I also don't
> see any difference in ubench's performance numbers.

Actually, I just upgraded this machine to 6.1 and now I am seeing much
the same thing - no matter what I set machdep.hyperthreading_allowed
and machdep.hlt_logical_cpu to I still see stuff on both CPUs. I havent
doe any performance measurements though.

-pete.