Xen and the Art of Virtualization

Xen is an x86 virtual machine monitor which allows multiple commodity operating systems to share conventional hardware in a safe and resource managed fashion, without sacrificing either performance or functionality. Xen is type I hypervisor, which directly runs on top of bare metal. We will summarize what Xen is what its attributes are.

paravirtualization - presents a virtual machine abstraction that is similar but not identical to the underlying hardware.

The Virtual Machine Interface

Memory is hard to virtualize mostly because x86 doesn’t support software-managed TLB. A tagged TLB entry allows both guest OS and hypervisor to coexist because it can be associated with an address-space identifier. This is not possible on x86, thus address space changing likely requires flushing the TLB. Thus, to achieve better performance, guest OSes are responsible to managing hardware page tables. Batching can be used by the guest OS to reduce constantly requesting new pages from the hypervisor when new processes are created.

CPU virtualization has implications for guest OSes. Principally, OS the most privileged entity on top of hardware. A hypervisor in the middle means the guests OSes must be modified to run a lower privilege level. On x86, this is not a problem...