Introduction to VMDirectPath

Although hardware independence is fundamental to many of the advantages of virtualization it also creates a problem when you need to connect a specific hardware device to a virtual machine. Although SCSI passthrough has been available in ESX for some time accessing other types of device, especially USB, has always required alternative solutions such as USB over IP hubs. With the release of vSphere4 VMware have introduced a new feature called VMDirectPath I/O which allows up to two PCI(e) devices on the host server to be connected to a Virtual Machine. Officially this is provided to reduce the latency and hence improve the performance of devices such as 10Gb NICs and Host Bus Adapters, in fact only a handful of such devices are supported by VMware. However in fact pretty much any PCI(e) device can be connected, just bear in mind that VMware will not help with any problems you may encounter

--> The rest is on :

http://www.petri.co.il/vmware-esxi4-vmdirectpath.htm