CS 695 Topics in Virtualization and Cloud Computing More Introduction
Process vs. System view of “machine”
●
ABI – application binary interface
Process Virtual Machine
System Virtual Machine
Virtual Machine Applications
System VM benefits
● Single application containers---reliability, isolation, security
● Mixed OS environments (legacy apps)
● Multi-platform application development
● Software testing and debugging
● Version transitioning
● Event monitoring and checkpointing
● Record-replay, migration of machines
● IaaS
Scope of course (revisited)
●
How to provide system-level virtualization?
● Common problems/approaches
● Specific techniques/optimizations
●
Assuming VMs exist (and can be flexed) ...
● How to exploit/use them to meet IaaS requirements?
Key VMM responsibilities
●
State management
– Switch VMs
●
Resource control
– VM has access to resources when scheduled
● cpu, memory region etc.
– VMM owns & controls all resources
●
VMM is required to control/arbitrate
hardware resources
Ex.1 : Process virtual memory example
VA to RA conversion
●
Each process provided with a (virtual) linear address space
● Translation via a system call
●
Number of page tables?
●
When process switches what happens?
Ex.2 : The interval time
●
interval timer
● OS (privileged) resource
● Set value to timer (register) when process scheduled
● Timer ticks to zero (time to deschedule process)
● Interrupt on regs value zero
● OS handles interrupt, schedules (new) process
●
How to manage interval timer with VMs?
● Should OS of VM be able to read/write interval timer?
● Write
● Read
Processor Virtualization
●
Different ISAs of guest and host
– Emulation
– Binary translation
●
Same ISAs of guest and host
– Direct native execution
– In all cases?
Virtualizability properties
●
Popek & Goldberg (1974)
– VMM should satisfy following properties
1. Efficiency
● Innocuous instructions natively executed 2. Resource control
● No direct control on physical resources for guest 3. Equivalence
● Identical behavior on native and virtual environments
● Performance and resource availability exceptions
Modes of execution
●
User mode
– Guest OS
– VMM
●
Privileged mode (System mode)
– VMM
Types of instructions
●
Privileged instructions
– Reduced functionality or no-permissions in user mode
– Generate trap when executed in user mode
– E.g.,
– LPSW (load processor status word from memory location)
● Set CPU mode, PC etc.
– SPT (set cpu timer)
Types of instructions
●
What should be condition on guest VMs?
Does this satisfy G&P conditions?
Does this satisfy G&P conditions?
●
Theorem 1:
– VMM can be constructed if, sensitive subset of priviliged
●
Theorem 2:
– VMM is virtualizable
What about problem cases?
●
Sensitive but unprivileged instructions
● Critical instructions
– 1. popf, pushf
● no-op for interrupt enable/disable in user mode
● sensitive instruction
– 2. %cs register
● Expose CPL to process (Guest OS in ring 3)
– 3. lar, lsl
● Load access rights, load segment limits
● Answer depends on privilege level
● Guest OS is at ring 3!
Handling Criticial instructions
●
Scan and patch
●
Paravirtualization
– Replace criticial instructions with new 'safe' versions
– New ISA exported by VMM
– Guest OS needs to re-implement/re-compile
●
Hardware-assisted virtualization
● AMD-V, VT-X
● Root and non-root modes
● Configure exit conditions
Resource virtualization - Memory
● What do Guest VMs expect?
– A linear memory address space
– “Real” addressable area, staring from 0x00000000
● Memory for processes
– Virtual memory (and paging)
Paging on Native Systems
●
cr3
●
page directory
●
page table
●
page
Paging on Native Systems
Resource virtualization - Memory
● What do Guest VMs expect?
– A linear memory address space
– “Real” addressable area, staring from 0x00000000
● Memory for processes
– Virtual memory (and paging)
● How to virtualize memory to meet these requirements?
Terminology
●
guest virtual address
– gfn : guest frame number
●
pseudo-physical address
– pfn : physical frame number
●
machine address
– mfn : machine frame number
(Two-level) translation
●
Virtual address of process in guest VM needs a real physical address
Pseudo-physical address
Guest virtual
address Machine
address
v2p p2m
Guest OS VMM
Direct Mapping
Shadow Paging
Hardware-assisted
Memory virtualization performance (1)
Memory virtualization performance (2)
What about the TLB?
●
Translation lookaside buffer
● v2m mapping in cache
●
When is TLB flushed?
●
Virtualizing the TLB
Next class
●
[kvm]
●
Next week
– [kvm]
– [xen]
