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Hardware-Assisted Virtualization
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Hardware-assisted virtualization enables virtualization software to use extra instructions available from the hardware to run guests. The extra hardware instructions eliminate the need for the virtualization software translate the instructions being used by the guest going to and back from the hardware. Eliminating the translation will result in better performance in your virtualization setup. To use hardware-assisted virtualization, you must have hardware that supports virtualization like the AMD Athlon 64 x2 and Intel s Core 2 Duo processors. Your virtualization software must be able to use the additional instructions provided by these processors, such as the Linux KVM (kernel-based virtual machine) (http://www.linux-kvm.org), and Xen-HVM (Xen s Hardware Virtual Machine).
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Operating System Virtualization
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In operating system virtualization, each guest has its own filesystem to store its tools, like shell commands and data, but each uses the kernel of the main host. Each guest that boots appears to
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CHAPTER 18 LINUX VIRTUALIZATION
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running its own kernel when sending instructions to the hardware. When a guest sends its instructions, the main host s kernel is actually the one sending those instructions to the hardware. The hardware uses the instructions and provides the required output back to the main host s kernel. The main host s kernel sends those instructions to the originating guest. You can view this approach as an inverted tree, where the root is the kernel that is being shared, and the guests are the branches as shown in Figure 18-2.
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Figure 18-2. The kernel being shared by the guests
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Note: See 17 to review kernel concepts.
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When you use this virtualization type, you have to make sure that the kernel can be shared by the guests. For example, if guests are created for the x86 architecture, the main host s kernel must be of the same architecture. Some examples of this type include OpenVZ (http://wiki.openvz.org) and LinuxVServer (http://www.linux-vserver.org).
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Paravirtualization
Paravirtualization uses a hypervisor or a virtual machine monitor to run guests. It uses two kinds of kernels, one that is built to be the hypervisor and the other for the guests to run. The hypervisor will be loaded on the main host and can be used to start guests. The guests will run their own kernels and boot as normal within the main host when started. When instructions need to be sent to hardware, guests send those to the hypervisor. The hypervisor will take care of the instructions and send them to the hardware. The instructions that the guests send to the hypervisor are called hypercalls. Hypercalls can be made only by guest kernels designed to interface with a hypervisor. Some examples that use this approach are Xen (http://www.xen.org) and Wind River (http://www.windriver.com). Paravirtualization makes use of its main host s available protection domains or protection rings when doing virtualization. Protection domains are an abstract concept used by operating systems to restrict processes to run according to their intended functions and use computer resources like memory and disk drives being accessed by those processes. Sometimes protection domains are implemented by
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CHAPTER 18 LINUX VIRTUALIZATION
the hardware itself such as the x86 architecture. This is also used to protect the main host from possible attacks by malware or wild processes that can crash the system. There are multiple protection domains on a system starting with 0 as the highest domain. Figure 183 shows the protection domains available on the x86 architecture.
Figure 18-3. The protection domains of an x86 architecture
The processes that run on ring 0 can use the hardware directly. This is where the operating system and hypervisors are used because they need to access the hardware. The Xen hypervisor needs to run on level 0 to use hardware that is requested by guests. The guests are run on ring 1, because they must not access hardware. Only by making hypercalls to the Xen hypervisor on ring 0 can the guests use hardware. User applications are run on ring 3, because they do not need to use hardware. Ring 2 is unused at present.
Note: For Xen, the hypervisor is called dom-0, short for domain 0. The kernel the guest uses is called dom-U,
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