Virtualizing the PC platform

Several technical and pragmatic hurdles must be overcome when virtualizing the PC platform. The traditional mainframe approach runs virtual machines in a less privileged mode in order to allow the VMM to regain control on privileged instructions, and relies on the VMM to virtualize and interface directly to the I/O devices. Also, the VMM is in complete control of the entire machine. This approach doesn't apply as easily to PCs for the following reasons.

Non-virtualizable processor  - The Intel IA-32 processor architecture [10] is not naturally virtualizable. Popek and Goldberg [11] showed that an architecture can support virtual machines only if all instructions that can inspect or modify privileged machine state will trap when executed from any but the most privileged mode. Because the IA-32 processor does not meet this condition, it is not possible to virtualize the processor by simply executing all virtual machine instructions in a less privileged mode.

PC hardware diversity  - There is a large diversity of devices that may be found in PCs. This is a result of the PC's ``open'' architecture. In a traditional implementation, the virtual machine monitor would have to manage these devices. This would require a large programming effort to provide device drivers in the VMM for all supported PC devices.

Pre-existing PC software  - Unlike mainframes that are configured and managed by experienced system administrators, desktop and workstation PC's are often pre-installed with a standard OS and set up and managed by the end-user. In this environment, it is extremely important to allow a user to adopt virtual machine technology without losing the ability to continue using his existing OS and applications. It would be unacceptable to completely replace an existing OS with a virtual machine monitor.

VMware Workstation has a hosted architecture that allows it to co-exist with a pre-existing host operating system, and rely upon that operating system for device support. Figure 2 illustrates the components of this hosted architecture. This architecture allows VMware to cope with the diversity of PC hardware and to be compatible with pre-existing PC software. Currently, Windows NT $^{\footnotesize\protect{\rm\ooalign{\hfil\raise.07ex\hbox{\sc r}\hfil\crcr\mathhexbox20D}}}$, Windows $^{\footnotesize\protect{\rm\ooalign{\hfil\raise.07ex\hbox{\sc r}\hfil\crcr\mathhexbox20D}}}$2000 and Linux can serve as hosts. This paper focuses on the performance aspects of relying on a host OS for accessing I/O devices.

Figure 2: VMware's hosted virtual machine model splits the virtualization software between a virtual machine monitor that virtualizes the CPU, an application the uses a host operating system for device support, and an operating system driver for transitioning between them.

The rest of this paper is organized as follows. Section 2 describes VMware Workstation's hosted architecture, its benefits and costs, and looks at the specific example of a virtual Ethernet network interface card (NIC). Section 3 demonstrates the performance of NIC virtualization with VMware Workstation 2.0, breaks down the overheads for a few different workloads, and measures improvements achieved by optimizations the data suggested. Section 4 presents several approaches for improving I/O performance beyond the optimizations described in Section 3, some of which go beyond the capabilities of a hosted architecture. Section 5 describes related work in the area of supporting multi-platform computing on a single machine. Finally, Section 6 summarizes the observed properties of the hosted architecture and draws some conclusions about this approach to I/O virtualization.