Xen and the beauty of virtualization BA ch 7

This chapter presents one of the hottest topics in information technology. Virtualization is increasingly becoming critical in today’s infrastructure because of its economic benefits and the features like isolation and security. Xen is an open source virtual machine monitor that allows multiples guess operating system to concurrently run on the same computer hardware under the supervision of the hypervisor.
One of the major challenge in architecting virtual machine monitors is to figure out how to executive privileged instructions in guess Operating system. This is because the hypervisor runs in the most privileged mode and the guess Os in lower privileged levels. In some systems, executing an instruction at an inappropriate privileged level causes an exception that the hypervisor can trap. However, this is not always possible because some system do not generate the exception.
Before Xen this issue was addressed by rewriting the OS code at runtime and replacing privileged instructions by direct calls to the hypervisor. The advantage of that approach is that The guess OS can run unmodified because emulation makes the virtual machine looks exactly the same as the physical hardware.
Xen introduced a new idea called paravirtualization, which presents an interface to the virtual machine that is not identical to the physical machine. The features of a machine that are hard to virtualize are replaced with hooks that allow the guess and the Host OS to communicate in executing the instruction. Paravirtualization remove the overhead of runtime code rewriting and emulation and can also allow some instruction to be executed directly by the guess OS. The only problem is that Guess OS need to be modified to support the paravirtualized architecture.

Xen’s architects designed the hypervisor to be a simple small layer between the hardware and the guess Operating systems, responsible for managing the hardware and scheduling Guess’s tasks and ensuring that each guess get a fair share of the hardware time. So to reduce complexity and the risk of damaging the whole system, most of the management functions are not done in the hypervisor, but at domain 0.

With the increasing use and importance of virtualization, most processor manufacturers are now providing virtualization support in their platform to allow unmodified guess operating systems to run on the hypervisor. The hardware is changed to notify the hypervisor each time a privileged instruction is executed. Latest version of Xen added support for these hardware virtual machines. The author makes the point that Xen was able to quickly provide this support because of its open source status. Developers from Intel and AMD helped write the low level code needed to supports the new CPU and Xen also used other open source projects like QEMU for device emulation. This is an example of some of the advantages of open source.