Virtualization
Virtualization allows concurrent execution of multiple operating systems and their applications on the same physical machine. Each OS thinks that it "owns" hardware resources but in reality the resources are virtual. Each operating system and its applications together are called a virtual machine.
Virtualization Layer
The classical definition of a virtual machine is an efficient, isolated duplicate of the real machine. VM si supported by a virtual machine monitor (VMM.)
Fidelity: VMM provides environment essentially identical with the original machine.
Performance: Programs should show at worst only minor decrease in speed.
Safety: VMM is in complete control of system resources.
Virtualization Models
There are two virtualization models.
Bare-metal
This is also called Hypervisor-based. VMM manages all hardware resoureces and supports execution of VM(s). It runs a service VM that runs a standard operating system and has full hardware priviledges to access and perform any kind of hardware manipulation. The service VM deals with devices and other configuration and management tasks.
Hosted
The other model is hosted virtualiation. In this model, at the lowest lefvel, there is a full fledged host operating system that manages all of the hardware resources. The host OS integrates a virtual machine monitor module, that's responsible for providing the virtual machines with their virtual platform interface and for managing all of the context switching scheduling and etc...
Processor Virtualization
Guest instructions are executed directly by the hardware. The virtual machine monitor does not interfere with every single instruction that is isseued by the guest operating system. As long as the guest operating system and its applications operate within the resources that were allocated to the VM, then everything is operating at its full hardware speed.
Whenever a priviledged instruction gets accessed, then the processor causes a trap, and control is automatically switched to the most priviledged level, so to the hypervisor.
Hypervisor will decide whether the operation is legal or illegal.
If illegal, terminate the VM
If legal, VMM emulates the behavior the guest OS was expecting from the hardware.
x86 Problems
However, a problem came up in the late 90s when x86 had 4 rings and no root/non-root mode. The hypervisor sits in ring 0 (highest priviledge mode) and guest OS sits in ring 1. Pre 2005, there were 17 priviledged instructions that do no trap and fail silently, e.g. interrupt enable/disable bit in priviledged register, POPF and PUSHF instructions that acess it from ring 1 would fail silently.
Hypervisor doesn't know so it doesn't try to change settings. OS doesn't know, so it assumes change was successful.
Binary Translation
VMWare pioneered the concept of binary translation. It dynamically performs binary translation.
Inspect code blocks to be executed
If needed, translate to alternate instruction sequence, e.g. to emulate desired behavior
Otherwise, run at hardware speeds
Memory Virtualization
Memory full virtualization has the following reuquirements.
All guests expect contiguous physical memory, starting at index 0
We distinguish three types of addresses
Virtual addresses
Physical addresses (the guest machine thinks it owns)
Machine addresses
Option 1
The guest page table maps virtual addresses to physical addresses. The hypervisor page table maps physical addresses to machine addresses.
Option 2
The guest page table maps virtual addresses to physical addresses. The hypervisor uses a shadow page table to map virtual addresses to machine addresses directly. It skips the translation layer between physical addresses to machine addresses. However, hypervisor needs to maintain consistency, e.g. invalidate on context switch, write-protect guest page table to track new mappings.
Memory paravitualization has the following implication.
Guest is aware of virtualization
No longer strict requirement on contiguous physical memory start at 0
Explicitly register page tables with hypervisor
Can batch page table updates to reduce VM exits
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