Hyperthread without BIOS enable

[James Wilkinson]

Rick Wagner wrote:
> I have a P4 system, which >>appears<< to be HT capable.  However, the BIOS
> (newest from vendor) does not give the option of turning HT on.  Without
> support from the BIOS, can HT be forced?  Or am I mis-reading the info below,
> and my CPU really is not HT capable.
> 
> BTW, I tested with 2.6.10-1.741_FC3smp, and tried "acpismp=force" (as
> described below).

Thiago Guzella wrote:
> Well, /proc/cpuinfo reports the HT feature flag as active, so this is
> a HT capable cpu...

Wouldn't it be nice? Some of Intel's documentation suggests this, too.

Unfortunately, Intel seems to consider that "hyperthreading" and
"actually having two logical CPUs" are different concepts.

References:
http://www.intel.com/support/processors/sb/cs-009861.htm and linked PDF
(500K): on page 17, it says:

> This field does not indicate that Hyper-Threading Technology has been
> enabled for this specific processor.  To determine if Hyper-Threading
> Technology is supported, check the value returned in EBX[23:16] after
> executing CPUID with EAX=1. If EBX[23:16] contains a value >1, then
> the processor supports Hyper-Threading Technology.

The value in EBX[23:16] is a count of the number of siblings.
Unfortunately, I can't see an easy way of getting at this without a C
(or assembler) program. This is right inside the processor: even a
changed BIOS couldn't change that. [1]

But http://indigo.intel.com/compare_cpu/default.aspx?familyID=1 lists
the 2.66 GHz Pentium 4 as not having hyperthreading.

This probably wasn't what you wanted to hear. Sorry.

James.

[1] Pointless speculation: *if* this just means that there is a working
but disabled second sibling logical processor in there, then conceivably
a "fixed" microcode could re-enable the processor. But you'd need *very*
good links with Intel to get such a thing.
-- 
James Wilkinson       | You will stop at nothing to reach your objective,
Exeter    Devon    UK | but only because your brakes are defective.
E-mail address: james | 
@westexe.demon.co.uk  |