Hyperthreading is not active

Hi,

The ESXi 5.5 (Build 9919047) running on Dell R710, and I enabled hyperthreading in Bios, however, ESXi show the status is "Disable (Enable on restart) (Not Active)", remark show "host BIOS must enable hyperthreading before the host can activate the feature., I just confuse that anything i missing?

Thanks,

Andy