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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 506
SEC. 7.2
REQUIREMENTS FOR VIRTUALIZATION
475
hypervisor to provide this illusion and to do it efficiently.
Indeed, hypervisors
should score well in three dimensions:
1.
Safety
: the hypervisor should have full control of the virtualized re-
sources.
2.
Fidelity
: the behavior of a program on a virtual machine should be
identical to that of the same program running on bare hardware.
3.
Efficiency
: much of the code in the virtual machine should run with-
out intervention by the hypervisor.
An unquestionably safe way to execute the instructions is to consider each in-
struction in turn in an
interpreter
(such as Bochs) and perform exactly what is
needed for that instruction. Some instructions can be executed directly, but not too
many. For instance, the interpreter may be able to execute an
INC
(increment) in-
struction simply as is, but instructions that are not safe to execute directly must be
simulated by the interpreter. For instance, we cannot really allow the guest operat-
ing system to disable interrupts for the entire machine or modify the page-table
mappings. The trick is to make the operating system on top of the hypervisor think
that it has disabled interrupts, or changed the machine’s page mappings. We will
see how this is done later.
For now, we just want to say that the interpreter may be
safe, and if carefully implemented, perhaps even hi-fi, but the performance sucks.
To also satisfy the performance criterion, we will see that VMMs try to execute
most of the code directly.
Now let us turn to fidelity. Virtualization has long been a problem on the x86
architecture due to defects in the Intel 386 architecture that were slavishly carried
forward into new CPUs for 20 years in the name of backward compatibility.
In a
nutshell, every CPU with kernel mode and user mode has a set of instructions that
behave differently when executed in kernel mode than when executed in user
mode. These include instructions that do I/O, change the MMU settings, and so
on. Popek and Goldberg called these
sensitive instructions
.
There is also a set of
instructions that cause a trap if executed in user mode.
Popek and Goldberg called
these
privileged instructions
.
Their paper stated for the first time that a machine
is virtualizable only if the sensitive instructions are a subset of the privileged in-
structions. In simpler language, if you try to do something in user mode that you
should not be doing in user mode, the hardware should trap.
Unlike the IBM/370,
which had this property, Intel’s 386 did not.
Quite a few sensitive 386 instructions
were ignored if executed in user mode or executed with different behavior.
For ex-
ample, the
POPF
instruction replaces the flags register, which changes the bit that
enables/disables interrupts. In user mode, this bit is simply not changed.
As a
consequence, the 386 and its successors could not be virtualized, so they could not
support a hypervisor directly.
Actually, the situation is even worse than sketched. In addition to the problems
with instructions that fail to trap in user mode, there are instructions that can read
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