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Modern Operating Systems by Herbert Bos and Andrew...
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Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 509
Type 1 hypervisor
(CPU, disk, network, interrupts, etc.)
(CPU, disk, network, interrupts, etc.)
Host OS
(e.g., Linux)
Excel Word Mplayer Emacs
Type 2 hypervisor
Guest OS
(e.g., Windows)
Guest OS process
Host OS
Figure 7-1.
Location of type 1 and type 2 hypervisors.
on the x86 market was
VMware Workstation
(Bugnion et al., 2012). In this sec-
tion, we introduce the general idea.
A study of VMware follows in Sec. 7.12.
Type 2 hypervisors, sometimes referred to as
hosted hypervisors
, depend for
much of their functionality on a host operating system such as Windows, Linux, or
When it starts for the first time, it acts like a newly booted computer and
expects to find a DVD, USB drive, or CD-ROM containing an operating system in
the drive. This time, however, the drive could be a virtual device. For instance, it is
possible to store the image as an ISO file on the hard drive of the host and have the
hypervisor pretend it is reading from a proper DVD drive.
It then installs the oper-
ating system to its
virtual disk
(again really just a Windows, Linux, or OS X file)
by running the installation program found on the DVD. Once the guest operating
system is installed on the virtual disk, it can be booted and run.
The various categories of virtualization we have discussed are summarized in
the table of Fig. 7-2 for both type 1 and type 2 hypervisors. For each combination
of hypervisor and kind of virtualization, some examples are given.
Virtualizaton method
Type 1 hypervisor
Type 2 hypervisor
Virtualization without HW support
ESX Server 1.0
VMware Workstation 1
Xen 1.0
Virtualization with HW support
vSphere, Xen, Hyper-V
VMware Fusion, KVM, Parallels
Process virtualization
Figure 7-2.
Examples of hypervisors. Type 1 hypervisors run on the bare metal
whereas type 2 hypervisors use the services of an existing host operating system.
Virtualizability and performance are important issues, so let us examine them
more closely.
Assume, for the moment, that we have a type 1 hypervisor sup-
porting one virtual machine, as shown in Fig. 7-3. Like all type 1 hypervisors, it

Page 510
SEC. 7.4
runs on the bare metal.
The virtual machine runs as a user process in user mode,
and as such is not allowed to execute sensitive instructions (in the Popek-Goldberg
sense). However, the virtual machine runs a guest operating system that thinks it is
in kernel mode (although, of course, it is not).
We will call this
virtual kernel
The virtual machine also runs user processes, which think they are in user
mode (and really are in user mode).
Type 1 hypervisor
Guest operating system
Virtual kernel mode
Virtual user mode
Trap on privileged instruction
User process
Figure 7-3.
When the operating system in a virtual machine executes a kernel-
only instruction, it traps to the hypervisor if virtualization technology is present.
What happens when the guest operating system (which thinks it is in kernel
mode) executes an instruction that is allowed only when the CPU really is in kernel
mode? Normally, on CPUs without VT, the instruction fails and the operating sys-
tem crashes.
On CPUs with VT, when the guest operating system executes a sensi-
tive instruction, a trap to the hypervisor does occur, as illustrated in Fig. 7-3. The
hypervisor can then inspect the instruction to see if it was issued by the guest oper-
ating system in the virtual machine or by a user program in the virtual machine.
the former case, it arranges for the instruction to be carried out; in the latter case, it
emulates what the real hardware would do when confronted with a sensitive in-
struction executed in user mode.
7.4.1 Virtualizing the Unvirtualizable
Building a virtual machine system is relatively straightforward when VT is
available, but what did people do before that? For instance, VMware released a
hypervisor well before the arrival of the virtualization extensions on the x86.
Again, the answer is that the software engineers who built such systems made
clever use of
binary translation
and hardware features that
exist on the x86,
such as the processor’s
protection rings
For many years, the x86 has supported four protection modes or rings. Ring 3
is the least privileged. This is where normal user processes execute. In this ring,
you cannot execute privileged instructions. Ring 0 is the most privileged ring that
allows the execution of any instruction. In normal operation, the kernel runs in

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