1. Virtualization
D. J. Foreman 2009

2. Reference
Formal Requirements for Virtualizable Third Generation Architectures, Gerald Popek and Robert Goldberg, CACM, V17 N7, July 1974
D. J. Foreman 2009

3. Advantages Resource configuration (RAM, devices, etc)
Run multiple (different) O/S’s concurrently
Test new releases
Isolation
Scalability (multiprocessor systems)
D. J. Foreman 2009

4. Modes of Operation Full simulation (NOT a true VMM) Hybrid VM
i.e.; instructions do not run natively
Hybrid VM
I/O resources are programmed by device drivers that run in a deprivileged service OS
VMM (often called a hypervisor)
large % of inst’s run natively
All privileged instructions trap to VMM for emulation
©D. J. Foreman 2009

5. Types Full Virt. Paravirtualization H/W assisted Guest unchanged
Generic device drivers
Paravirtualization
Guest can access host h/w
Modified drivers
H/W assisted
VMM moved to h/w
Requires virtualization h/w
D. J. Foreman 2009

6. VMM Management Allocator - resource management
Dispatcher - determines post-trap action
Simulator
performs the instruction
limits effects to the VM
VM sees only its own machine
©D. J. Foreman 2009

7. “Sensitive” i86 Instructions (in protected mode)
Control – can change amount of resources without causing a trap
Behavior – execution depends on real-memory location or mode
Includes:
IN - Input
OUTS - Output String
INS - Input String
CLI - Clear Interrupt-Enable Flag (IF)
OUT - Output STI - Set IF
©D. J. Foreman 2009

8. Privileged i86 Instructions
include:
CLTS - Clear Task-Switched Flag
LMSW - Load Machine Status
HLT - Halt Processor
LTR - Load Task Register
LGDT - Load GDT Register
MOV CRn - Move Control Register
LIDT - Load IDT Register
MOV DRn - Move Debug Register
LLDT - Load LDT Register
MOV TRn - Move Test Register
©D. J. Foreman 2009

9. V86 mode sensitive Inst’s
include:
CLI - Clear IF
POPF - Pop Flags
STI - Set IF
INT n - Software Interrupt
PUSHF - Push Flags
IRET - Interrupt Return
POPF is both IOPL and CPL sensitive. IOPL will only be changed by POPF when executed at CPL = 0. The interrupt flag (IF) will only be changed by POPF if executed with CPL <= IOPL. If POPF does not change IOPL or the interrupt flag due to lack of privilege, no exception is generated.
©D. J. Foreman 2009

10. VMM Types Type I runs directly on real hardware. Type II Hybrid
runs as an application on a host OS
relies on the host OS for
memory management,
processor scheduling, resource allocation, and
hardware drivers.
When a VM traps by attempt to execute a sensitive instruction, the host OS must direct the signal to the VMM
Hybrid
interprets every privileged instruction
©D. J. Foreman 2009

11. X86 Problems There are Sensitive, Unprivileged instructions
Do not cause an interrupt or exception!
Do not perform intended actions
Guest does not know they “failed”
©D. J. Foreman 2009