Understanding VT-d: Intel Virtualization Technology for Directed I/O

I've received a fair number of questions on the Intel Virtualization and Software Development Forum about VT-d -- enough to justify a few words on this note in the blogging sphere.

"VT-d" stands for "Intel Virtualization Technology for Directed I/O". The relationship between VT and VT-d is that the former is an "umbrella" term referring to all Intel virtualization technologies and the latter is a particular solution within a suite of solutions under this umbrella.

The overall concept behind VT-d is hardware support for isolating and restricting device accesses to the owner of the partition managing the device.

A VMM may support various models for I/O virtualization, including emulating the device API, assigning physical I/O devices to VMs, or permitting I/O device sharing in various manners. The key problem is how to isolate device access so that one resource cannot access a device being managed by another resource.

VT-d, at the time of this writing, includes four key capabilities

1. I/O device assignment. This feature allows an administrator to assign I/O devices to VMs in any desired configuration.

2. DMA remapping. Supports address translations for device DMA data transfers.

3. Interrupt remapping. Provides VM routing and isolation of device interrupts.

4. 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

Click on the link entitled "Intel Virtualization Technology (Intel VT) for Directed I/O (Intel VT-d) Architecture Specification". This brings up a detailed guide that should answer a broad range of technical questions.

David Ott

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