Lecture15-16.1 – Virtualisation Professor. Sinnott
Director, eResearch University of Melbourne
• Virtualisation – Motivation
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– What happens in a VM?
– Historical perspective
– Requirements for virtualisatio – Virtualisation approaches
– Memory management
– Live migration
Motivation
• Server Consolidation
– Increased utilisation
– Reduced energy consumption
• Personal virtual machines can be created
– No hardware purchase needed – Public cloud computing
• Security/Isolation
– Share a single machine with multiple users
• Hardware independence – Relocate to different hardware
Terminology
• VirtualMachineMonitor/Hypervisor: The virtualisation layer between the underlying hardware (e.g. the physical server) and the virtual machines and guest operating systems it supports.
– The environment of the VM should appear to be the same as the physical machine
– Minor decrease in performance only
– Appears as though in control of system resources
• VirtualMachine:Arepresentationofa real machine using hardware/software that can host a guest operating system
• GuestOperatingSystem:Anoperating system that runs in a virtual machine environment that would otherwise run directly on a separate physical system
VMM/Hypervisor
Recap on Kernel-User mode separation
User Space
Process 1 Process 2
Context switch Context switch
Kernel Space
Operating System Kernel
• Processes run in lower privileged (user) mode
• OS Kernel runs in privileged Kernel mode
• OS typically virtualises memory, CPU, disk etc giving appearance of complete access to CPU/memory/disk to application processes
– Each process has illusion of access to some/all of the memory or the CPU (but actually shared across multiple processes)
• Context switches can catch (trap) “sensitive” calls
– e.g. add two numbers vs change bios settings;
– Sensitive calls -> instruction sets are typically device specific, e.g. ARM vs x86 vs …
What Happens in a VM?
VMM/Hypervisor
Gives perception of a whole machine
Host Network Device
Virtual Network Device
Image file on disk (vmdk, vhd, qcow2 etc)
Virtual Disk
Host hard drive
VHD (Virtual Hard Disk represents a virtual hard disk drive (HDD). May contain what is found on a physical hard disk, such as disk partitions and a file system, which in turn can contain files and folders.
VMDK (Virtual Machine Disk) describes containers for virtual hard disk drives to be used in virtual machines like VMware.
qcow2 (QEMU Copy On Write) file format for disk image files used by
QEMU. It uses a disk storage optimization strategy that delays allocation of storage until it is actually needed.
• Guest OS apps “think” they write to hard disk but translated to virtualised host hard drive by VMM
• Which one is determined by image that is launched
Historical Perspective
• Virtualisation concept goes back to 1960s
• IBM System/370
• System/370 featured hardware support for interpretive execution
• Formal requirements laid down by Popek and Goldberg (1974)
• Why then?
– Mainframes needed ability to run multiple kinds of applications
• Hardware was very expensive “back in the day”
Origins – Principles
“an efficient, isolated duplicate of the real machine” Properties of interest
– Fidelity:SoftwareontheVMM executes behaviour identical to that demonstrated when running on the machine directly, barring timing effects
– Performance:Anoverwhelming majority of guest instructions executed by hardware without VMM intervention
– Safety:TheVMMmanagesall hardware resources
Communications of the ACM, vol 17, no 7, 1974, pp.412-421
Classification of Instructions
• Privileged Instructions: instructions that trap if the processor is in user mode and do not trap in kernel mode
• Sensitive Instructions: instructions whose behaviour depends on the mode or configuration of the hardware
– Different behaviours depending on whether in user or kernel mode
• e.g. POPF interrupt (for interrupt flag handling)
• Innocuous Instructions: instructions that are neither privileged nor sensitive
– Read data, add numbers etc
Origins – Principles
• Theorem (Popek and Goldberg)
– For any conventional third generation computer, a virtual machine
monitor may be constructed if the set of sensitive instructions for
that computer is a subset of the set of privileged instructions • i.e. have to be trappable
Example of Privilege Rings
• Ring 0: Typically hardware interactions
• Ring 1: Typically device drivers
• Specific gates between Rings (not ad hoc)
• Allows to ensure for example that spyware
can’t turn on web cam or recording device etc
• Significance
– The IA-32/x86 architecture was not originally virtualisable
x86 Virtualisability
• x86 architecture was historically not
virtualisable, due to sensitive instructions that
could not be trapped, e.g. instructions such as: • SMSW – storing machine status word
• SGDT, SLDT – store global/local descriptor table register • POPF – interrupt flag (user/kernel mode)
– Robin and Irvine, Analysis of an Intel Pentium’s Ability to Support a Secure Virtual Machine Monitor, Usenix, 2000
• Intel and AMD introduced extensions to make x86 virtualisable
– AMD SVM (Secure Virtual Machine) – Intel VT (Virtualisation Technology)
• These are very similar, but use slightly different machine instructions
Typical Virtualisation Strategy
Trap and Emulate!
• VMM emulates the effect on system/hardware resources of privileged instructions whose execution traps into the VMM
– aka trap-and-emulate
• Typically achieved by running GuestOS at a lower hardware priority level than the VMM
• Problematic on some architectures where privileged instructions do not trap when executed at de-privileged level
VMM needs to support:
– De-privileging
privileged instruction
emulate change change
– Primary/shadow structures
• VMM maintains “shadow” copies of critical structures whose “primary” versions are manipulated by the GuestOS, e.g. memory page tables
• Primary copies needed to ensure correct versions are visible to GuestOS
– Memory traces
• Controlling access to memory so that the shadow and primary structure remain coherent
• Common strategy: write-protect primary copies so that update operations that might cause page faults can be caught, interpreted, and addressed
– Someone’s app/code doesn’t crash the server you
are using!!!
Major VMM and Hypervisor Providers
VMM Provider
VM Architecture
VMWare Workstation
x86, x86- 64
x86, x86-64
Windows, Linux
Windows, Linux, Solaris, FreeBSD, OS/2
Full Virtualization
VMWare ESX Server
x86, x86- 64
x86, x86-64
No Host OS
Same as VMWare workstation
Baremetal hypervisor
x86, x86- 64, IA-64
x86, x86- 64, IA-64
NetBSD, Linux, Solaris
Windows, Linux, Solaris, FreeBSD, OS/2, NetBSD
Para- virtualisation
Linux, Windows, FreeBSD, Solaris
Hardware virtualisation
Aspects of VMMs
• Full virtualisation – allow an unmodified guest OS to run in isolation by simulating full hardware (e.g. VMWare)
– Guest OS has no idea it is not on physical machine vs
• Para-virtualisation – VMM/Hypervisor exposes special interface to guest OS for better performance. Requires a modified/hypervisor- aware Guest OS (e.g. Xen)
– Can optimise systems to use this interface since not all instructions need to be trapped/dealt with
Full Virtualisation
• Advantages
– Guest is unaware it is executing within a VM – Guest OS need not be modified
– No hardware or OS assistance required
– Can run legacy OS
Direct execution of code
• Disadvantages
– can be less efficient
User/kernel split typically
• VMM run Ring 0
• Apps run in Ring 3
Virtualisation (Guest OS) uses extra rings; VMM traps privileged instructions and translates to hardware specific instructions
Ring 3 Ring 2
Ring 1 Ring 0
Binary translation of OS privileged
Host Hardware
Para-Virtualisation
• Advantages
– Lower virtualisation overheads, so better performance, e.g. Xen
• Disadvantages
– Need to modify guest OS • Can’t run arbitrary OS!
– Less portable
– Less compatibility
Ring 3 Ring 2
Ring 1 Ring 0
Direct execution of code
Calls to virtualisation layer replace non-virtualisable instructions
Paravirtualised Guest
Host Hardware
Aspects of VMMs (contd…)
• Hardware-assisted virtualisation – Hardware provides architectural support for running a Hypervisor (e.g. KVM)
– New processors typically have this
– Requires that all sensitive instructions trappable
• Binary Translation – Trap and execute occurs by scanning guest instruction stream and replacing sensitive instructions with emulated code (e.g. VMWare)
– Don’t need hardware support, but can be much harder to achieve
• Rarely ever 1:1 mapping between instruction sets
Hardware-assisted virtualisation
• Advantages
– Good performance
– Easier to implement
– Advanced implementation supports hardware assisted DMA, memory virtualisation, …
• Disadvantages
– Needs hardware support
Ring 3 Ring 2
Ring 1 Ring 0
Direct execution of code
Calls to virtualisation layer replace non-virtualisable instructions
-1 supported
Page tables, virtual memory mgt, direct memory access for high-speed reads etc
Host Hardware
Binary Translation
• Advantages
– Guest OS need not be modified
– No hardware or OS assistance required
– Can run legacy OS
• Disadvantages
– Overheads
– Complicated
– Need to replace instructions “on-the- fly”
– Library support to help this, e.g. vCUDA
Ring 3 Ring 2
Ring 1 Ring 0
Direct execution of code
Binary translation of OS privileged
Host Hardware
Aspects of VMMs (contd…)
• Bare Metal Hypervisor – VMM runs directly on actual hardware (e.g. VMWare ESX Server)
– Boots up and runs on actual physical machine – VMM has to support device drivers, all HW mgt
• Hosted Virtualisation – VMM runs on top of another operating system (E.g. VMWare Workstation,…)
Operating System Level Virtualisation
• Lightweight VMs (containers)
• Instead of whole-system virtualisation, the
OS creates mini-containers
– A subset of the OS is often good enough for many use cases
• Can’t use for running Windows on Linux etc, but often not needed!
– Akin to an advanced version of “chroot”
• operation that changes apparent root directory for current running process and subprocesses. Program run in such a modified environment cannot access files and commands outside that environmental directory tree.
• Examples:
– LXC, Docker, OpenVZ, FreeBSD Jails etc
Operating System Level Virtualisation
• Advantages
– Lightweight
– Many more VMs on same hardware
– Can be used to package applications and all OS dependencies into container
• Disadvantages
– Can only run apps designed for the same OS – Cannot host a different guest OS
– Can only use native file systems
– Uses same resources as other containers
Memory Virtualisation
• Conventionally page tables store the logical page number -> physical page number mappings
– Seems like more memory than actually have
• What happens in a VM?
Shadow Page Tables
VMM machine
OS physical
• VMM maintains shadow page tables in lock-step with the page tables
• Adds additional management overhead
• Hardwareperforms guest -> physical and physical -> machine translation
process virtual
Shadow Page Table Entry
“shadow” page tables
page tables
Live Migration from Virtualisation Perspective
Source: Clark et al, Live migration of virtual machines, NSDI 2005
Clark et al. Live migration of virtual machines, NSDI 2005.
Effects of Live Migration
Source: Clark et al, Live migration of virtual machines, NSDI 2005
References
• Hwang, Dongarra & Fox, 2011. Distributed and Cloud Computing, 1st Edition. Elsevier.
• &. 2005. Virtual machine monitors: Current technology and future trends, in IEEE Computer.
• Clark et al, 2005. Live migration of virtual machines. In Proceedings of the 2nd ACM/USENIX Symposium on Networked Systems Design and Implementation (NSDI). pp. 273-286
• K. Adams, O. Agesen, 2006. A Comparison of Software and Hardware Techniques for x86 Virtualization, ASPLOS 2006
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