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Implementing Virtualization: Page 3 of 7

From a processor standpoint, the challenge was to provide a processor-level framework for sharing devices, now and into the future, and Intel and AMD have created very similar specifications to address this. Both specs were released in spring 2006, and virtualization vendors have already signed on to support both.

AMD's IOMMU (I/O memory mapping unit) technology offers additional instructions to support hardware virtualization. These new features are designed to improve DMA (direct memory access) mapping and access for hardware devices, replace the current mechanism for graphics addressing, and support direct control of devices by a VM while enabling device direct access to user-space I/O within a VM.
Intel's VT-d (Virtualization Technology for Directed I/O) standard was released at almost the same time as IOMMU and focuses on the same problems of direct device access and memory protection. Like IOMMU, VT-d will provide a framework for direct communication between VMs and I/O devices on the Intel platform.

These features will have only a nominal impact on virtualization at first, because the other half of the solution for I/O virtualization is still a ways off. At present, there are no I/O devices capable of managing shared VM access to hardware resources. In fact, there isn't even a standard in place for device sharing across PCI, and it's likely to be two to four years before we see common, device-based hardware solutions for I/O virtualization. Until that time, virtualization vendors will need to provide an abstraction layer that supports shared access to device drivers for storage, network and other interrupt-driven devices.

This particular challenge is in the process of being addressed right now by the PCI-SIG, the international governing body for the PCI system interface. The IOV spec now under consideration will provide a reference standard for efficient virtual device sharing that should begin appearing in IO products like high-performance HBAs and NICs in the 2008/2009 timeframe. See "IOV: The Final Frontier of Server Virtualization".

Everyone's a Winner
Of the three main hypervisor technologies that are rising to the top of the evolutionary chain, newer offerings from Xen and Microsoft will get the biggest leg up from chip-assisted virtualization. These new processor-level capabilities eliminate many of the major stumbling blocks that VMware spent years resolving via impressively clever software workarounds. As hardware virtualization improves, it's becoming easier and easier to take creative approaches to these problems, and the focus is moving away from the hypervisor itself toward performance and management issues.