FreeBSD Security Advisory FreeBSD-SA-05:09.htt [REVISED]

[Colin Percival]

Chris wrote:
> I am somewhat confused by applying the patch, does this disable HTT
> functionality? or does a patched server close the issue and keep HTT
> enabled?

The patch adds a new loader tunable, machdep.hyperthreading_allowed,
which is set to 0 by default.  Regardless of how you try to set the
machdep.hlt_cpus_mask and machdep.hlt_logical_cpus sysctls, if
machdep.hyperthreading_allowed is set to zero then you will not
have any process threads executing on the second hyper-thread from
each core.

If you're on a system where this isn't a problem (e.g., anything with
no local users), you can set machdep.hyperthreading_allowed=1 in
/boot/loader.conf or via the sysctl after booting, and get the benefit
of hyperthreading.

Colin Percival