Disabling hyperthreading on Intel CPUs

[andy thomas <andy%time-domain.co.uk@localhost>]

Dos anyone know what happned to the machdep.hyperthreading_allowed 
tunable? It was definitely there in (I think) 5.x but in 8.1 it's not:

        andy@dell380:~ $ sysctl machdep.hyperthreading_allowed=0
        sysctl: unknown oid 'machdep.hyperthreading_allowed'

There are some new CPU sysctls in 8.x but they seem to disable cores 
rather than hyperthreading, which is enabled by default in the kernel. I 
would like to be able to turn off hyperthreading at runtime on a large 
number of HP PCs that are fitted with single quad-core CPUs without 
touching the BIO settings.

Andy