Re: is it em64t ?
On Sat, Jan 07, 2006 at 12:45:04PM +0100, Ernest jw ter Kuile wrote:
> On Saturday 07 January 2006 11:46, Hamish Moffatt wrote:
> >
> > It does, but what if HyperThreading was present but disabled in the
> > BIOS? I don't know how you would tell in that case.
>
> Most of these flags are not set simply because the kernel found a certain type
> of processor. They are the result of elaborate startup tests which detect
> these capabilities.
>
> If a capability is disabled in the bios, two things can happen
>
> 1) the kernel does not detect it at all; in which case the relevant flag is
> not set;
>
> 2) the kernel detects the capability but switched off; in which case the
> kernel will attempt to switch it on (if it can of course). If this succeeds
> the flag is set, if not you will often see an error and the flag is not
> switched on.
>
> I have been told, however, that a few processor capabilities are so bound to
> the a certain type of cpu that their flag is simply set if that cpu is found.
>
> I don't know under which category the hyperthreading flag falls, but knowing
> that hyperthreading can be disabled in the bios, I would not expect the
> kernel to simply set that flag blindly.
I think that's all wrong. The flags are more or less a decoding of the
CPUID result codes.
As for HT, my dad's old laptop 1.7GHz P4-mobile (Northwood) has the ht
flag, but it sure as hell doesn't have two logical CPUs. I looked at (but
didn't really come close to understanding ;P) the relevant kernel code, and I
think the ht flag indicates support for an API for asking how many logical
CPUs there are, and so on, not that there actually are multiple logical CPUs
on that physical CPU.
The kernel log messages (look in /var/log/dmesg if other messages have
bumped them from the ring buffer) are more useful than the ht flag.
I don't remember if x86info is useful for this or not, but it's generally
good at finding out about CPUs.
--
#define X(x,y) x##y
Peter Cordes ; e-mail: X(peter@cor , des.ca)
"The gods confound the man who first found out how to distinguish the hours!
Confound him, too, who in this place set up a sundial, to cut and hack
my day so wretchedly into small pieces!" -- Plautus, 200 BC
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