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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
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 540
SEC. 7.12
CASE STUDY: VMWARE
509
CPU
VMM Context
Host OS Context
Kernel mode
User mode
Disk
int handler
int handler
IDTR
Any
Proc.
Host OS
write()
fs
scsi
VMM
Driver
world
switch
VMM
VMX
Virtual Machine
(i)
(ii)
(iii)
(iv)
(v)
Figure 7-10.
The VMware Hosted Architecture and its three components: VMX,
VMM driver and VMM.
These components each have different functions and operate independently
from one another:
1. A user-space program (the
VMX
) which the user perceives to be the
VMware program. The VMX performs all UI functions, starts the vir-
tual machine, and then performs most of the device emulation (front
end), and makes regular system calls to the host operating system for
the back end interactions. There is typically one multithreaded VMX
process per virtual machine.
2. A small kernel-mode device driver (the
VMX driver
), which gets in-
stalled within the host operating system. It is used primarily to allow
the VMM to run by temporarily suspending the entire host operating
system. There is one VMX driver installed in the host operating sys-
tem, typically at boot time.
3.
The VMM, which includes all the software necessary to multiplex the
CPU and the memory, including the exception handlers, the trap-and-
emulate handlers, the binary translator, and the shadow paging mod-
ule. The VMM runs in kernel mode, but it does not run in the context
of the host operating system. In other words, it cannot rely directly on
services offered by the host operating system, but it is also not con-
strained by any rules or conventions imposed by the host operating
system. There is one VMM instance for each virtual machine, created
when the virtual machine starts.
Page 541
510
VIRTUALIZATION AND THE CLOUD
CHAP. 7
VMware Workstation appears to run on top of an existing operating system,
and, in fact, its VMX does run as a process of that operating system. However, the
VMM operates at system level, in full control of the hardware, and without de-
pending on any way on the host operating system.
Figure 7-10 shows the relation-
ship between the entities: the two contexts (host operating system and VMM) are
peers to each other, and each has a user-level and a kernel component. When the
VMM runs (the right half of the figure), it reconfigures the hardware, handles all
I/O interrupts and exceptions, and can therefore safely temporarily remove the host
operating system from its virtual memory. For example, the location of the inter-
rupt table is set within the VMM by assigning the IDTR register to a new address.
Conversely, when the host operating system runs (the left half of the figure), the
VMM and its virtual machine are equally removed from its virtual memory.
This transition between these two totally independent system-level contexts is
called a
world switch
.
The name itself emphasizes that everything about the soft-
ware changes during a world switch, in contrast with the regular context switch im-
plemented by an operating system.
Figure 7-11 shows the difference between the
two. The regular context switch between processes ‘‘A’’ and ‘‘B’’swaps the user
portion of the address space and the registers of the two processes, but leaves a
number of critical system resources unmodified. For example, the kernel portion of
the address space is identical for all processes, and the exception handlers are also
not modified. In contrast, the world switch changes everything: the entire address
space, all exception handlers, privileged registers, etc. In particular, the kernel ad-
dress space of the host operating system is mapped only when running in the host
operating system context. After the world switch into the VMM context, it has
been removed from the address space altogether, freeing space to run both the
VMM and the virtual machine. Although this sounds complicated, this can be im-
plemented quite efficiently and takes only 45 x86 machine-language instructions to
execute.
Host OS
Context
Process
A
Process
B
Normal
Context Switch
VMware
World Switch
VMM
Context
Linear Address space
VMM
A (user-space)
Kernel Address space
B (user-space)
VMX (user-space)
Kernel Address space
Kernel Address space (host OS)
Virtual Machine
Figure 7-11.
Difference between a normal context switch and a world switch.
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