I'd installed a Win XP Pro x86_64 guest on a Fedora 13 x86_64 KVM host. The host had 8 hyperthreaded CPUs (16 virtual cores in total). I had assigned all 16 vcpus to the WinXP guest, but it only showed 2 in the task manager, though in the device manager, it showed 16. The problem had to do with licensing: XP Pro is limited to 2 physical CPUs (even though it can see 16 cpus in the device manager). It can support (at least) 4 cores per cpu however. So the solution is to specify this in the config, which I found (via googling) can be done by modifying the xml defining the guest. To find the layout of the host:
virsh capabilities | grep topology
<topology sockets='2' cores='4' threads='2'/>
Using 'virsh edit <domain>', find where the vcpus are set:
<vcpu>16</vcpu>
Then add the following below:
<cpu>
<topology sockets='2' cores='4' threads='2'/>
</cpu>
Save it, and restart the guest. It now sees all vcpus.
I notice that from looking at the xml, libvirt reorders it slightly:
<vcpu>16</vcpu>
<os>
<type arch='x86_64' machine='pc-0.13'>hvm</type>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
<pae/>
</features>
<cpu>
<topology sockets='2' cores='4' threads='2'/>
</cpu>
