Similarly, the processor is supposed to have 8 total threads (per the same spec page linked above), but when I use the recommended cpuid call:
I get 16 threads instead of 8.
I'd heard that it was possible to infer similar information using CPUID.0Bh (thus accessing the x2apic processor topology functions), and while CPUID.0.EAX does yield "11", I also found reference that one could check if CPUID.1.ECX to see if x2apic was enabled, but on my processor that bit is 0. Sure enough, calls to CPUID.0Bh with any value of ECX have EAX=0x0 and EBX=0x0.
As to the EAX=0xB failure, __cpuid may be rejecting 'InfoType' it doesn't know about. I would recommend another means of accessing CPUID, particularly because it doesn't seem possible to set ECX otherwise (how are you doing that?).