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The Xen VMM. Nathanael Thompson and John Kelm. Motivations. Full virtualization of x86 is hard and imperfect Instead, make hosted OS aware of virtualization but not hosted applications Enable performance isolation and accounting. Outline. Motivations The Xen Implementation

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the xen vmm

The Xen VMM

Nathanael Thompson

and

John Kelm

motivations
Motivations
  • Full virtualization of x86 is hard and imperfect
  • Instead, make hosted OS aware of virtualization but not hosted applications
  • Enable performance isolation and accounting
outline
Outline
  • Motivations
  • The Xen Implementation
  • Performance Evaluation
  • Xen Extensions
  • Discussion topics
paravirtualization design goals
Paravirtualization: Design Goals
  • Modified OS, unmodified applications
  • Leverage OS knowledge of virtualization to provide high-performance VM
  • Enable hosting of 10’s-100’s of VM’s on a single machine
paravirtualization vs full virtualization

Control

Plane

User

Apps

Guest OS

Dom0

Xen

Paravirtualization

Paravirtualization vs. Full Virtualization

User Applications

Ring 3

Ring 2

Ring 1

Binary Translation

Guest OS

VMM

Ring 0

Full Virtualization

paravirtualization implementation
Paravirtualization: Implementation
  • Key Point: Make changes to OS
    • Paging issues: updates and faults

(40% of hypervisor time, says Intel)

    • Optimize access virtual devices (I/O rings)
    • Provide fast/batch call mechanisms via hypercalls
  • Hide Xen in top of each VM address space—similar to VMWare (maybe?)
problematic instructions
Problematic Instructions

Guest OS

1. Privileged Instruction

HLT

CLI

STI

RTI

5. Return From Interrupt

Xen

(Normal Context)

Instr Handler

3. Trap and…

4. …Emulate Instruction

Xen

(Interrupt Context)

GPF Handler

Why not just paravirtualize?

GPF

x86 Protection Mechanisms

2. Protection Fault!

domain 0
Domain 0
  • Put control/VMM interface, real device drivers, etc. into a separate VM
  • Sets up new VMs—Could use for migration?
  • Why not just put this all into Xen proper?
    • Increased difficulty in proving isolation
    • Larger footprint (Remember where Xen is located in virtual memory)
    • Fewer services available inside hypervisor
    • Take advantage of guest OS driver API
memory management
Memory Management
  • Avoid shadow page tables, but we have to trust OS, right?
  • Batch updates for performance gain—Hypercalls to the rescue!
  • Page frame types: PT’s, DT’s, RW…why?
  • OS manipulates page tables—Is this safe?
porting an os to xen
“Porting” an OS to Xen
  • Modify OS to run in x86 ring 1
  • Replace or trap sensitive instructions with equivalent without overhead of binary translation: Do the dynamic translation statically
  • Hypercalls to make direct transfer from GuestOS to Xen
  • Paper describes Linux port—about 3,000 LoC added
  • WinXP port did not materialize: Politics? Technical difficulties?
communication interfaces
Communication Interfaces
  • Xen runs virtual firewall-router
    • Domain0 sets rules for firewall
    • Performs NAT
    • Isolates traffic between domains
  • I/O rings for both transmit and receive
    • Outgoing packets sent in round robin order
      • Xen copies packet header, but not data - for safety. Why?
    • Guest provides page frame for each incoming packet - no copying
performance single app
Performance: Single App
  • Outperforms VMWare on most user-mode and OS benchmarks—was this a fair comparison?
  • Performance on user-mode benchmark applications nearly identical to native Linux—is this surprising?
  • OS performance close to native, but page manipulation (e.g. mmap, PF) still has high cost
  • Pathological benchmarks showed process isolation in native Linux not as strong as VM isolation in Xen
  • Singal handling in XenoLinux lower latency than native! How could this happen?
xenoservers an application of xen
XenoServers: An Application of Xen
  • Distributed platform for running untrusted code
  • Applications move between servers based on location, system load, cost, etc.
  • Virtual machine allows complex server configurations + isolation + accounting = Xen
  • Reed, et al. “Xenoservers: Accountable Execution of Untrusted Programs”, HotOS ‘99
live vm migration
Live VM Migration
  • Moving OS keeps kernel state
  • Moving entire OS removes (some) residual dependencies
  • Basic approach:
    • Reserve resources on new machine
    • Copy pages
    • Commit
    • Activate
  • Clark, et al. “Live Migration of Virtual Machines”, NSDI 2005
live vm migration1
Live VM Migration
  • Iterative pre-copy
    • Copy all memory pages
    • Then copy those dirtied during the last round
  • In order to finish Stop-and-Copy phase halts OS and copies final pages
  • 50-210ms downtime for various servers
  • Network? Disk?
    • same LAN segment and network attached storage
xen with intel vt
Xen with Intel VT
  • Intel Performance Analysis :
    • 40% of hypervisor time spent on paging
    • Shared Mem. FB provided 5-1000x speedup on X
    • PIC caused VM exits from I/O ports when scheduling timers—pushed it into hypervisor
xen with intel vt cont
Xen with Intel VT (cont.)

Is this a more fair comparison of VMs?

What does it tell us about HW-assisted full virtualization versus paravirtualization?

practical questions
Practical Questions?
  • When is it worthwhile to "port" an OS to Xen? Three years later and no XP port, only works with added HW support.
  • Does Xen really isolate VM's? If I compromise the guest, have I compromised the host?
  • What does binary translation buy us?
    • Dynamic optimization
    • Higher overhead
    • May not work for all situations well on many architectures
philosophical questions
Philosophical Questions?
  • Should Xen be in the mainline kernel tree?
  • Do we need standard VM API?
  • Does VirtualPC already use some amount of paravirtualization for Windows OSes?
further discussion topics
Further Discussion Topics
  • Can performance isolation be achieved without paravirtualization?
  • Are evaluations convincing?
  • How does one measure a VMM?
guest os i o interface
Guest OS I/O Interface
  • I/O Port, I/O mmap, I/O channel partitioning
  • Virtual devices
    • Fast networking possible—inter-VM can be made very fast, but there is a problem…
    • May not have source for driver (e.g., nv Linux driver)
    • State issues
  • I/O Rings
    • Latency and throughput issues?
guest os i o interface cont
Guest OS I/O Interface (cont.)
  • Strive for zero-copy transfers
  • Block Device Accesses
    • Leverage OS/VM interaction to prioritize access—what about isolation?
    • Block caching schemes/block sharing
    • DMA Issues with contiguous physical regions and pinned memory? IOMMU’s?