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SubjectRe: hyperthreading
Måns Rullgård writes:
> Gábor Lénárt <lgb@lgb.hu> writes:
>
> >> > flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca
> >> > cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm
> >> > bogomips : 3334.14
> >>
> >>
> >> P4 does not support HT ... only Xeon and new generation P4-HT does.
> >
> > OK, but if this CPU does not support HT, then why 'ht' is shown at
> > flags in /proc/cpuinfo? It looks like quite illogical for me then ...
>
> I've been wondering about that myself. I'm sure my P4M doesn't have
> any hyperthreading.

Don't people read Intel docs any more?

- The HT feature flag means "might have HT, check further to make sure".
- "check further" involves checking #siblings >= 2 via CPUID.
Old P4s typically announce HT but have #siblings < 2.
- Intel made HT enabling a hardware thing via the chipset (a pin
sampled at reset). BIOS can disable HT but only HW can enable it.
This is why you can't put a HT-capable P4 in a non-HT-chipset
machine and expect it to work -- although PowerLeap seemed to
have an adapter to "fix" that.
-
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