Ashtyn Hemendinger Natalie Mark Hong Ding Joseph Minjae Lee

 Platform Virtualization: technique used to implement a certain kind of virtual machine environment: one that provides a complete simulation of the underlying hardware.  In full virtualization, the virtual machine simulates enough hardware to allow an unmodified "guest" OS (one designed for the same instruction set) to be run in isolation.

 Hardware-assisted virtualization was first introduced on the IBM System/370 in 1972, for use with VM/370, the first virtual machine operating system. Virtualization was eclipsed in the late 1970s, with the advent of minicomputers that allowed for efficient timesharing, and later with the commoditization of microcomputers.

 To compensate for these architectural limitations, virtualization of the x86 architecture has been accomplished through two methods: full virtualization or paravirtualization. Both create the illusion of physical hardware to achieve the goal of operating system independence from the hardware but present some trade-offs in performance and complexity.  Well-known implementations of hardware- assisted x86 virtualization include VMware Workstation (for 64-bit guests only), Xen 3.x (including derivatives like Virtual Iron), Linux KVM and Microsoft Hyper-V.

 Hardware-assisted virtualization reduces the maintenance overhead of paravirtualization as it restricts (ideally, eliminates) the amount of changes needed in the guest operating system. It is also considerably easier to obtain better performance. A practical benefit of hardware-assisted virtualization that has been cited by VMware engineers and Virtual Iron.

 Hardware-assisted virtualization requires hardware support, which has only recently (2007) become available on x86 processors.  A “pure” hardware-assisted virtualization approach, using entirely unmodified guest operating systems, involves many VM traps, and thus high CPU overheads; this limits scalability and the efficiency of server consolidation.

 High CPU overheads can be mitigated by the use of paravirtualized drivers; the combination has been called “hybrid virtualization”.  Though there are lots of setbacks and defects, virtualization has potential to be very useful in the future when the technology becomes established.