Hyperthreading issues.

[Doug White]

On Fri, 11 Nov 2005, kama wrote:

> Just upgraded from 5.4 to 6.0 and hyperthreading stoped working.
> Everything looks ok, but it doesn't use two of the logical CPU's.

This is disabled by default due to a information-leak vulnerability across
the hyperthreaded cores. The details from the release notes:

Because of an information disclosure vulnerability on processors using
Hyper-Threading Technology (HTT), the machdep.hyperthreading_allowed
sysctl variable has been added. It defaults to 1 (HTT enabled) on FreeBSD
CURRENT, and 0 (HTT disabled) on the 4-STABLE and 5-STABLE development
branches and supported security fix branches. More information can be
found in security advisory FreeBSD-SA-05:09.htt. [MERGED]

If you don't care about this, add

machdep.hyperthreading_allowed="1"

to /boot/loader.conf and reboot.

> One other thing is when I try to switch off hyperthreading in BIOS, it
> will hang at bootup when it are settling the scsi drives. After awhile it
> will give me scsi timeouts. This only happens when I have two cpu enabled
> and hyperthreading off. If I disable one cpu w ht off it will boot wo
> problems, or two cpus w ht. But booting with ht + two cpu's gives me the
> other problem.

Sounds like the BIOS is not rerouting the interrupts correctly. Check for
a BIOS update.

-- 
Doug White                    |  FreeBSD: The Power to Serve
dwhite at gumbysoft.com          |  www.FreeBSD.org