When a Xen
server boots up, it first starts the hypervisor, which is responsible for starting a
domain named Domain0 (dom0) in which the host operating system runs. dom0 is
a privileged domain that can access the hardware resources on the server and also
contains tools for managing all the other domains. We will learn how to run Xen in
the next chapter. The hypervisor also checks the page tables and is responsible for the
allocation of resources for new domains. Each domain interacts with the hypervisor
by giving it a hypercall, which is a software trap from the domain to the hypervisor.
The hypervisor responds to the hypercall by sending an event to the domain.
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Introduction
[ 8 ]
New instances of unprivileged domains or virtual machines are created by using the
tools that are available in dom0. These tools in turn make calls to the control interface
API in the hypervisor to perform the requested operations. These unprivileged
domains are referred to as domU.
For instance, if you are running a Xen server with three guests, you will have dom0,
dom1, and dom2 as the domains on that server. The dom0 is identical to the other
domU??™s, except that it has all the hardware access. We will learn how to create and
use new domU instances in Chapter 3.
All requests from the domU instances for hardware access are made to the back-end
device drivers in dom0 that will pass on these requests to the actual hardware.
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