On Tue, 04 Jan 2011 12:10:19 +0100, Peter Zijlstra said:

> X86_FEATURE_HT tells us the CPU supports telling us about HT,
> smp_num_siblings > 1 tells us the CPU is capable of HT.

A subtle but important distinction. Several laptops ago, I had a P4-M chip that
had FEATURE_HT, but num_siblings == 1.  ISTR trying to boot an SMP kernel on it
and it dying gloriously trying to bring up the non-existent other sibling.