While VT-d enables the direct assignment of devices to guest VMs, it does not directly facilitate the efficient sharing of devices across multiple guest VMs. Such efficient sharing is not feasible without fundamental changes in the way that devices present their resources to the platform. Further work is being done in the PCI-SIG [21] [22] to enhance the PCI Express* specifications to enable devices to be shared.

Briefly, these extensions enable PCI Express devices to support multiple virtual functions, each of which can be discovered, configured, and managed. This allows the direct assignment of a virtual function to a VM using VT-d, thus allowing a single physical device to be sharable among multiple VMs.

The importance and applicability of these sharable PCI Express devices may be largely dependent upon the performance requirements, usage model, and market segment in which they may be deployed.