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(or wrong)—yet—we think we do right by exploring the similarity between hyper-
visors and microkernels a bit more.
The main reason the first hypervisors emulated the complete machine was the
lack of availability of source code for the guest operating system (e.g., for Win-
dows) or the vast number of variants (e.g., for Linux).
Perhaps in the future the
hypervisor/microkernel API will be standardized, and subsequent operating sys-
tems will be designed to call it instead of using sensitive instructions. Doing so
would make virtual machine technology easier to support and use.
The difference between true virtualization and paravirtualization is illustrated
in Fig. 7-5. Here we have two virtual machines being supported on VT hardware.
On the left is an unmodified version of Windows as the guest operating system.
When a sensitive instruction is executed, the hardware causes a trap to the hypervi-
sor, which then emulates it and returns.
On the right is a version of Linux modified
so that it no longer contains any sensitive instructions. Instead, when it needs to do
I/O or change critical internal registers (such as the one pointing to the page
tables), it makes a hypervisor call to get the work done, just like an application pro-
gram making a system call in standard Linux.
Unmodified Windows
Modified Linux
Trap due
to sensitive
Trap due
to hypervisor
True virtualization
Type 1 hypervisor
Figure 7-5.
True virtualization and paravirtualization
In Fig. 7-5 we have shown the hypervisor as being divided into two parts sepa-
rated by a dashed line. In reality, only one program is running on the hardware.
One part of it is responsible for interpreting trapped sensitive instructions, in this
case, from Windows. The other part of it just carries out hypercalls. In the figure
the latter part is labeled ‘‘microkernel.’’ Ifthe hypervisor is intended to run only
paravirtualized guest operating systems, there is no need for the emulation of sen-
sitive instructions and we have a true microkernel, which just provides very basic
services such as process dispatching and managing the MMU. The boundary be-
tween a type 1 hypervisor and a microkernel is vague already and will get even less
clear as hypervisors begin acquiring more and more functionality and hypercalls,
as seems likely.
Again, this subject is controversial, but it is increasingly clear that
the program running in kernel mode on the bare hardware should be small and reli-
able and consist of thousands, not millions, of lines of code.

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