Operating-Systems-Notes

Virtualization

Virtualization allows concurrent execution of multiple OSs and their applications on the same physical machine.

• Virtual resources : each OS thinks that ot “owns” hardware resources
• Virtual machine (VM) : OS + applications + virtual resources (guest domain)
• Virtualization layer : management of physical hardware (virtual machine monitor, hypervisor)

Defining Virtual Machine

A Virtual Machine is an efficient, isolated duplicate of the real machine.

• Supported by a Virtual Machine Monitor (VMM):
  1. provides environment essentially identical with the original machine
  2. programs show only minor decrease in speed at worst
  3. VMM is in complete control of the system resources

VMM goals

• Fidelity
• Performance
• Safety and Isolation

Virtualization advantages

• consolidation
  • decrease cost, improve manageability
• migration
  • availibility, reliability
• security, debugging, support for legacy OS

Two main Virtualization Models:

1. Bare-metal or Hypervisor based (Type 1)

• VMM (hypervisor) manages all hardware resources abd supports execution of VMs
• privileged, secure VM to deal with devices (and other configuration and management tasks)
• Adopted by Xen(Opensource or Citriol Xen Server) and ESX (VMware)

1. Hosted (Type 2)

• Host owns all hardware
• Special VMM modle provides hardware interfaces to VMs and deals with VM context switching

Virtualization requirements

• Present virtual platform interface to VMs
  • virtualize CPU, memory, devices
• Provide isolation across VMs
  • preemption, MMU for address translation and validation
• Protect guest OS from applications
  • can’t run guest OS and applications at same protection level
• Protect VMs from guest OS
  • can’t run guest OS and VMMs at same protection level

Hardware protection levels

Commodity hardware has more than two protection levels

• x86 has 4 protection levels (rings)
  • ring 3 : lowest privilege (applications)
  • ring 1 : OS
  • ring 0 : highest privilege (hypervisor)
• and 2 protection modes
  • non root : VMs - ring 3 : apps
    • ring 0 : OS
  • root : - ring 0 : hypervisor

Process Virtualization (Trap-and-Emulate)

• Guest instruments
  • executed directly by hardware
  • for non-privileged operations : hardware speeds => efficiency
  • for privileged operations : trap to hypervisor
• Hypervisor determines what needs to be done:
  • if illegal operation : terminate VM
  • if legal operation : emulate the behaviour the guest OS was expecting from the hardware

Problems with Trap-and-Emulate

• 17 privileged information do not trao but fail silently
• Hypervisor doesn’t know, so it doesn’t try to change settings
• OS doesn’t know, so assumes change was successful

Binary Translation

Goal : Full Virtualization i.e. guest OS is not modified

Approach : Dynamic Binary Translation

1. Inspect code blocks to be executed
2. If needed, translate to alternate instruction sequence
   • e.g. to emulate desired behaviour, possibly avoid traps
3. Otherwise run at hardware speeds
   • cache translated blocks to ammortize translation costs

Paravirtualization

Goal : Performance; give up on modified guest OSs

Approach : Paravirtualization : modify guest OSs so that

• it knows it is running virtualized
• it makes explicit calls to hyperisor (hypercalls)
• hypercalls (~ system calls)
  • package context information
  • specify desired hypercall
  • trap to VMM
• Xen : opensource hypervisor

Memory virtualization

• Full virtualization
  • all guests expect contiguous physical memory starting at 0
  • virtual vs physical vs machine addresses and page frame numbers
  • still leverages hardware (MMU, TLB..)
• Option 1
  • guest page table : VA => PA
  • hypervisor : PA => MA
  • too expensive!
• Option 2
  • guest page tables : VA => PA
  • hypervisor shadow PT : VA => MA
  • hypervisor maintains consistence - e.g. invalidate on context switch, write protect guest PT to track new mappings
• Paravirtualized
  • guest aware of virtualization
  • no longer strict requirement on contiguous physical memory starting at 0
  • explicitly registers page tables with hypervisor
  • can “batch” page tables updates to reduce VM exits
  • other optimazations

Overheads eliminated or reduced on newer platforms

Device Virtualization

• For CPUs and Memory
  • less diversity, Intruction-Set-Architecture(ISA) level
  • Standardization of interface
• For Devices
  • high diversity
  • lack of standard specification of device interface and behaviour

3 key models for Device Virtualization:

1. Pass through model

Approach: VMM-level-driver configures device access permissions

Advantages

• VM provided with exclusive and direct (VMM bypass) access to the device

Disadvantages

• Device sharing difficult
• VMM must have exact type of device as what VM expects
• VM migration tricky

2. Hypervisor - Direct model

Approach:

• VMM interrupts all device accesses
• Emulate device operations
  • translate to generic I/O operations
  • traverse VMM-resident I/O stack
  • invoke VMM-resident driver

Advantages

• VM decoupled from physical device
• Sharing, migration, dealing with device specifics

Disadvantages

• Latency of device operations
• Device driver ecosystem complexities in Hypervisor

3. Split Device-Driver model

Approach:

• Device access control split between
• Emulate device operations
  • front-end driver in guest VM (device API)
  • back-end driver in service VM (or Host)
  • modified guest drivers - i.e. limited to paravirtualized guests

Advantages

• Eliminate emulation overhead
• Allow for better management of shared devices

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