How do you detect that hyperthreading is activated?

["Björn Antonsson" <d93-ban@nada.kth.se>]

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Hi,
I have an application that needs to check if a linux kernel has booted
with hyperthreading enabled on ia32, in order to do some special tweaks.
I know how you detect that a CPU supports hyperthreading, but how do you
detect that the kernel has booted with hyperthreading activated?

And by hyperthreading activated I mean that the current number of CPUs,
for example 8, are actually 4 physical CPUs and 4 logical CPUs.

Is there a way that I can get at the logical/physical CPU bits, as
reported by the CPUID instruction, of all available CPUs? Or is there
an even better way?

/Björn