Deste excelente post no blog da Intel que está no topo dos resultados do google para vt-d
:
VT-d, at the time of this writing, includes four key capabilities
I/O device assignment. This feature allows an administrator to assign I/O devices to VMs in any desired configuration.
DMA remapping. Supports address translations for device DMA data transfers.
Interrupt remapping. Provides VM routing and isolation of device interrupts.
Reliability features. Reports and records system software DMA and interrupt erros that may otherwise corrupt memory of impact VM isolation.
Note that VT-d is not dependent on VT-x. That is, a VT-x enabled system can operate without VT-d, or without VT-d enabled or configured. You simply miss the benefits of the feature. Many people have asked about this point.
You can read about VT-d in detail at:
http://www.intel.com/technology/virtualization/technology.htm?iid=tech_vt+tech
Este link é a especificação arquitetônica do VT-d, se você tiver uma tarde para gravar e realmente quiser saber como ela funciona.