[CentOS] smp falls back to up mode on quad core

Hi,

I'm running Centos 5.1 on an Intel Quad 2 Core with four Q6600 processors.
Despite Centos 5 supporting multiple cores (SPM), it doesn't seem to work
for me:
1) a dmesg shows that only one processor is seen:
     "SPM alternatives: switching to UP code", etc.
2) file /proc/cpuinfo shows only one processor
3) top with option 1 (to toggle between seeing all processors or an average)
shows only 1 processor.

I've googled this, and other people have found this kind of trouble, but I
was not able to find any solutions.

By the way: I've installed windows xp to see if it was a bios problem, but
windows sees the 4 processors just fine.

Any ideas?

Thanks,

Julian.
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