Questions: Virtual Machines and Hypervisors

Question 1

A student runs Oracle VirtualBox on a Windows laptop to boot Linux in a window. Which description fits this setup?

• A. A Type 1 hypervisor, because it runs a full operating system
• B. A Type 2 hypervisor, because it runs as an application on a host operating system
• C. A container, because both systems share one kernel
• D. A paravirtualized system, because Linux must be modified

Answer: (B)

VirtualBox is a hosted (Type 2) hypervisor: Windows is the host OS and VirtualBox runs on top of it. A Type 1 (bare-metal) hypervisor, such as VMware ESXi or Xen, runs directly on the hardware without a general-purpose host OS beneath it.

Question 2

Under the classic trap-and-emulate approach, what happens when a guest kernel, running in a less privileged CPU mode, executes a privileged instruction such as one that disables interrupts?

• A. The instruction runs on the real CPU and disables interrupts for the whole machine
• B. The CPU traps to the hypervisor, which applies the effect to the virtual machine's state only
• C. The guest kernel crashes, because guests cannot use privileged instructions
• D. The instruction is silently ignored

Answer: (B)

The hypervisor runs the guest kernel without full privilege. A privileged instruction therefore causes a trap into the hypervisor, which emulates its effect on the VM's virtual CPU, for example marking that VM's virtual interrupts as disabled, and then resumes the guest.

Question 3

Each Linux container on a host runs its own separate operating system kernel, just as each virtual machine does.

• T. True
• F. False

Answer: False

Containers share the host kernel. The kernel gives each container its own view of processes, files and network (namespaces) and limits its resource use (cgroups). This makes containers start faster and use less memory than VMs, but they offer a weaker isolation boundary and cannot run a different kernel.

Question 4

In 1974, Popek and Goldberg gave a condition for building an efficient trap-and-emulate hypervisor. The original 32-bit x86 architecture did not meet it. State the condition and explain why x86 failed it.

Model answer: The set of sensitive instructions (those that read or change privileged machine state) must be a subset of the privileged instructions, so that every sensitive instruction traps when run outside kernel mode. Original x86 had sensitive instructions that did not trap in user mode; they behaved differently or failed silently, so a hypervisor could not catch them.

Designers worked around this with binary translation (rewriting problem instructions, as early VMware did) and paravirtualization (modifying the guest, as Xen did). Hardware support added in the mid-2000s, Intel VT-x and AMD-V, introduced a new CPU mode that makes trap-and-emulate work on x86.