[CentOS] SMP motherboard not recognized

I'm having trouble with IBM xseries 345/dual Xeon 2.66GHz showing only one processor. At boot I see "SMP motherboard not recognized". Using

title CentOS (2.6.18-8.1.8.el5xen) root (hd0,0) kernel /xen.gz-2.6.18-8.1.8.el5 module /vmlinuz-2.6.18-8.1.8.el5xen ro root=/dev/VolGroup00/LogVol00 rhgb quiet module /initrd-2.6.18-8.1.8.el5xen.img

have tried acpi=no (causes boot-hang), acpi=ht (boot hang), noapic (no effect), and noht (no effect)

Booting to gentoo-minimal CDrom shows expected 4 cores (dual HT) with cat /proc/cpuinfo, only 1 using 2.6.18-8.1.8.el5xen

Any ideas? many thanks