Intel Virtualization Technology for Directed I/O (Intel VT-d)

Intel® Virtualization Technology for Directed I/O (Intel® VT-d) 

Introduction

Intel's holistic approach to virtualization has resulted in several hardware assists throughout the platform, which provide simpler and more robust means for virtualization and in many cases reduction in software overheads associated with virtualization. This article describes the enhancements to the I/O device virtualization that Intel® Virtualization Technology (Intel® VT) for Directed I/O (Intel® VT-d) offers and takes a look at the overall platform-level I/O virtualization approach by Intel.

The importance of I/O virtualization

As virtualization usage models gain prominence in the data center, I/O device virtualization becomes increasingly important to achieve a complete platform virtualization solution. These usage models require isolation between a virtual machine (including the I/O resources assigned to it) and other virtual machines (VMs). Isolation prevents VM operations from interfering with one another, which is essential for achieving availability, reliability, and trust in virtual environments.

As IT consolidates server workloads, more VMs will run on servers, with a growing number of I/O devices to support them. Approaches to I/O device virtualization based on software provide flexibility in terms of I/O sharing and hardware transparency. However, software emulation can result in lower performance as I/O throughput increases, due to the overhead inflicted by the virtual machine monitor (VMM) to emulate the devices. Neither can software virtualization of I/O devices control or monitor direct memory access (DMA) by I/O devices to physical memory, creating a significant challenge to isolating VMs and their assigned I/O devices. But, hardware assistance in concert with system software can provide the mechanism to isolate DMA activities.

Hardware support for I/O device virtualization allows system software to securely assign particular I/O devices to VMs directly. Direct assignment with hardware assistance removes the VMM's I/O emulation layer, which can result in throughput improvement for a VM. As devices that can natively share their resources with multiple VMs become available, direct assignment capability will be further enhanced as more VMs can be reliably and directly assigned to a single device.

Intel VT-d seeks to provide the necessary hardware assistance to make the I/O device virtualization safer, simpler, and more robust.

Intel Virtualization Technology for Directed I/O overview

Intel Virtualization Technology (Intel VT) for Directed I/O (Intel VT-d) extends Intel VT roadmap for providing hardware assistance for virtualization solutions. Intel VT-d continues from the existing support for IA-32 (Intel® VT-x) and Intel® Itanium® p