1. Virtual Machine Monitors
CSE451
Andrew Whitaker

2. Hardware Virtualization
Running multiple operating systems on a single physical machine
Examples:
VMWare, Microsoft’s VirtualPC / VirtualServer, Parallels (Macintosh), Xen

3. Virtual Machine Monitors
Type I VMM runs on the raw hardware
Type II VMM runs hosted on another OS
Windows
virtual machine
Linux
virtual machine
virtual machine monitor
hardware

4. VMM History Conceived by IBM in the late 1960’s
Popularized by VM/370 (1972)
Used for OS debugging, time sharing, supporting multiple OS’s
Batch processing OS
Time sharing OS
VM/370
System 370 Machine

5. VMMs Today OS development and debugging Software compatibility testing
Running software from another OS
Or, OS version
Virtual infrastructure for Internet services

6. Internet Services Many applications now run inside the Internet
Search engines, maps, photo sharing, , IM, game servers, etc.
Goal: allow anyone to upload to a new service into the Internet
Must be low-cost, secure, robust, easy to maintain, etc.
Approach: use VMMs to provide a rent-a-server economy

7. Amazon’s Elastic Compute Cloud (EC2)
Provide service developers with a set virtual machines and storage resources
Scalability
New machines can be created in minutes
Security
Virtual machines provide stronger isolation than OS processes
Developer control
Developers choose the OS, software, libraries, etc.
Low cost
Developers pay only for what they use

8. VMM Implementation Overview
A VMM is just an operating system that exposes a (virtual) hardware interface
Windows
virtual machine
Linux
virtual machine
virtual
architecture
Virtual machines safely shared. Even if the system has one disk, we need the appearance of N virtual disks; N TLBs; N
Processors; N Ethernet devices, etc. =
virtual machine monitor
physical
architecture
hardware

9. Comparing the Unix and VMM APIs
Storage
File system
(virtual) disk
Networking
Sockets
(virtual) Ethernet
Memory
Virtual Memory
(virtual) Physical memory
Display
/dev/console
(virtual) Keyboard, display device

10. Possible Implementation Strategy: Complete machine emulation
The VMM implements the complete hardware architecture in software
while(true) {
Instruction instr = fetch();
// emulate behavior in software
instr.emulate(); }
Drawback: This is really slow

11. Review: Protection in Traditional OS’s
Hardware exposes a user/kernel boundary
Operating system runs in kernel mode
Processes run in user mode
Processes can safely execute most instructions
No need to “emulate” machine behavior
But, a small set of instructions can only execute in privileged mode
e.g., writing to an I/O device, disabling interrupts, etc.

12. Improving VMM Performance
Treat guest operating systems like an application
Most instructions execute natively on the CPU
Privileged instructions must be trapped and emulated
Virtual machines
Physical hardware
loads,stores,
branches,
ALU operations
Systems lore: make the common case fast, and the uncommon case correct
machine halt,
I/O instructions,
MMU manipulation
VMM

13. Handling Privileged Instruction
Virtual machine issues a privileged instruction (e.g., disk read)
VMM determines whether the virtual machine was in “user” mode or “kernel” mode
Note: the virtual mode is distinct from the physical mode (Yikes!)
If “user” mode, raise a protection exception
If “kernel” mode, emulate the disk read in software; then, return control to the guest OS

14. Tracing Through a File System Read
Application
Guest OS
VMM
read() syscall
trap handler
handle read syscall
read from disk()
trap privileged instruction
If “kernel” mode:
emulate virtual disk
else:
raise protection violation
finish read syscall
copy data to user buffer
return from system call
return from read()

15. Virtual Disk: Possible Implementations
Static disk partitions
A file in the file system
Especially for type-II VMMs
A special virtual disk file system
A network storage abstraction
e.g., Amazon’s S3
Use the layer of indirection to provide new services. For example, we could provide cryptographic storage, even if the OS/ file system do not implement this behavior

16. Virtualizing the User/Kernel Boundary
Both the guest OS and applications run in (physical) user-mode
This is necessary so that privileged instructions trap into the VMM
For each virtual machine, the VMM keeps a software mode bit:
During a system call, switch to “kernel” mode
On system call return, switch to “user” mode
Note: A faster implementation is possible on x86

17. (Virtual) Physical Memory
Each guest OS expects its own page tables, TLB, memory-management unit, etc.
Unlike the virtual disk, we can’t trap and emulate every memory reference
WAY too slow
Additional complication: protection bits
Some memory can only be accessed in “kernel” mode

18. Memory Model Virtual Memory (Applications) Increasing privilege
Physical Memory
(Guest OS)
Machine Memory
(VMM)
Assume a software-loaded TLB:
InsertTLB(int virtualPageNumber,int physPageNumber);

19. Virtual Physical Memory, Implementation
(Bad) Option #1: Insert the V-to-P mapping directly into the TLB; Trap each memory access to return the V-to-M memory
Better Option #2: Instead of inserting a V-to-P mapping, transparently insert a V-to-M mapping
This way, all future memory references will occur at hardware speed

20. Performance Details
TLB must be flushed on a process switch and a virtual machine switch
Whenever a virtual machine regains the processor, it requires numerous page faults to restore its virtual memory mappings
Optimization: VMM maintains a cache of V-to-M mappings for each virtual machine
On taking a page fault, the VMM can insert the appropriate TLB mapping without consulting the guest OS