It is well known that virtualization of the x86 architecture is an old idea. The Intel 386 processor (1985) introduced the “Virtual 8086” (V86) mode, enabling users to run real-mode operating systems as a task within a 32-bit protected-mode operating systems.
A more complete virtualization of the x86 architecture which includes 16-bit and 32-bit protected mode is likewise relatively old. One of the better known products which provided full x86 virtualization on x86 systems, VMware Workstation, dates back to 1999. Emulation on other architectures is even older, such as Virtual PC for the PowerPC Mac (1997).
On x86 architecture hosts, virtualization had to contend with numerous “holes” in the 32-bit x86 instruction set which made virtualization difficult and/or slow. Some of the more common issues are the POPF instruction which may quietly fail to update the interrupt flag, or the SMSW instruction which lets the guest operating system see the true state of control bits without allowing the hypervisor to trap it. To overcome these and other issues, Intel designed the VT-x (also known as VMX) extension to the x86 architecture, and AMD developed its own AMD-V hardware virtualization support. Specifications for VT-x and AMD-V were only published in 2005 and 2006, respectively; it took several more years for x86/x64 CPUs with hardware virtualization support to become mainstream. Yet the idea of complete x86 hardware virtualization is much, much older—by more than 15 years! Continue reading →
