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Xen and the Art of Virtualization. Paul Barham * , Boris Dragovic , Keir Fraser, Steven Hand, Tim Harris, Alex Ho, Rolf Neugebauery , Ian Pratt, Andrew Wareld * Microsoft Research Cambridge , UK University of Cambridge Computer Laboratory

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xen and the art of virtualization

Xen and the Art of Virtualization

Paul Barham*, Boris Dragovic, Keir Fraser, Steven Hand, Tim Harris,Alex Ho, Rolf Neugebauery, Ian Pratt, Andrew Wareld

*Microsoft Research Cambridge, UK

University of Cambridge Computer Laboratory

19th ACM Symposium on Operating System principles(SOSP’03)

  • Resurgence of interest in VM technology(2003)
    • Modern computers are sufficiently powerful to use virtualization.
  • In this paper we present Xen:
    • a high performance resource-managed virtual machine monitor(VMM)
problem need to solve
Problem need to solve
  • VM isolation:
    • It is not acceptable for the execution of one to adversely affect the performance of another.
  • Different operating systems enabled:
    • To accommodate the heterogeneity of popular applications.
  • Performance overhead:
    • the performance overhead introduced by virtualization should be small.
xen approach overview
  • Traditional approach— full virtualization:
    • The virtual hardware exposed is functionally identical to the underlying machine.
    • Benefit:
      • allowing unmodified operating systems to be hosted.
    • Drawback:
      • Support for full virtualization was never part of the x86 architectural design.
xen approach overview cont d
  • Improvement:
    • Paravirtualization:
      • Presenting a virtual machine abstraction that is similar but not identical to the underlying hardware.
      • Improved performance.
      • Require modifications to the guest operating system.
        • But no modifications are required to guest applications.
          • ABI: an application binary interface (ABI) describes the low-level interface between an application (or any type of) program and the operating system or another application.
the virtual machine interface
The Virtual Machine Interface
  • Overview of the paravirtualized x86 interface:
    • Memory management
    • CPU
    • Device I/O
  • Why x86?
    • x86 represents a worst case
memory management
Memory management
  • Software-managed TLB V.S. Physical-managed TLB
  • Two decision:
    • To ensure safety and isolation:
      • Guest OSes are responsible for allocating and managing the hardware page tables
      • Minimal involvement from Xen;
    • Avoiding a TLB flush when entering and leaving the hypervisor.
      • Xen exists in a 64MB section at the top of every address space.
memory management cont d
Memory management(cont.d)
  • Method:
    • A guest OS requires a new page table
      • EX: a new process is being created.
    • It allocates and initializes a page from its own memory reservation and registers it with Xen.
    • Guest OS relinquish direct write privileges to the page-table memory.
      • All subsequent updates must be validated by Xen
    • Note:
      • Guest OSes may batch update requests to amortize the overhead of entering the hypervisor.
  • In order to paravirtualize CPU, the hypervisor must have higher privilege level than guest OS.
    • Prevents the guest OS from directly executing privileged instructions
      • Isolation.
      • EX: memory management design we discuss before.
    • In x86, processor has 4 privilege levels in hardware.
      • The x86 privilege levels are generally described as rings.
        • From ring 0 to ring 3(0 is the most privileged)
      • Therefore, hypervisor is set to ring 0, guest OS is set to ring 1, ring 3 is set to applications.
        • Any OS which follows this common arrangement can be ported to Xen by modifying it to execute in ring 1.
cpu cont d
  • Exception handle:
    • EX: page faults and software exception.
    • A table describing the handler for each type of exception is registered with Xen for validation.
      • Overhead.
      • Safety is ensured by validating exception handlers.
        • Validate the handler's code segment does not specify execution in ring 0.
cpu cont d1
  • Exception handle(cont.d):
    • To deal with overhead:
      • only two types of exception occur frequently enough to affect system performance
        • system calls (usually implemented via a software exception)
        • page faults(no solution)
      • System calls can be registered to a `fast' exception handler.
        • Accessed directly by the processor without indirecting via ring 0.
device i o
Device I/O
  • Full-virtualized environments
    • Emulating existing hardware devices
  • Paravirtualized:
    • Xenexposes a set of clean and simple device abstractions.
      • Objective: Protection and isolation.
    • I/O data is transferred to and from each VM via Xen.(describe later)
      • In order to perform validation checks
        • EX: checking that buffers are contained within a domain's memory reservation.
detail design
Detail Design
  • Control Transfer:
    • Hypercallsand Events
  • Data Transfer:
    • I/O Rings
  • Subsystem Virtualization
    • CPU scheduling
    • Virtual address translation
    • Network
    • Disk
control transfer hypercalls and events
Control Transfer: Hypercalls and Events
  • Hypercalls:
    • Synchronous calls from a VM to Xen.
      • In order to perform a privilege operation
      • EX: VM request a set of page table updates
  • Events:
    • Notifications are delivered to VM from Xen using an asynchronous event mechanism.
      • Replaces the usual delivery mechanisms for device interrupts.
      • EX: Indicate that new data has been received over the network.
      • Guest OS may specify an event-callback handler to respond to the notification.
control transfer hypercalls and events1
Control Transfer: Hypercalls and Events
  • Events(cont.d):
    • Pending events:
      • Stored in a per-domain bitmask which is updated by Xen.
    • How to pend events?
      • Set a Xen-readable software flag.
        • This is analogous to disabling interrupts on a real processor.
data transfer i o rings
Data Transfer: I/O Rings
  • Data transfer mechanism main idea
    • Allows data to move vertically through the system with as little overhead as possible.
    • Minimize the work required to demultiplexdata to a specific VM when an interrupt is received from a device
data transfer i o rings1
Data Transfer: I/O Rings
  • I/O data buffers are allocated out-of-band by the guest OS
    • Zero copy:by transfer the pointer and edit permission.





data transfer i o rings2
Data Transfer: I/O Rings
  • Order:
    • There is no requirement that requests be processed by Xen.
      • The guest OS associates a unique identifier with each request which is reproduced in the associated response.
      • Reason: reorder I/O operations due to scheduling or priority considerations.
cpu scheduling
CPU scheduling
  • Scheduling alg:
    • Borrowed Virtual Time (BVT) scheduling algorithm[11]
      • work-conserving
      • has a special mechanism for low-latency wake-up when VM receives an event

[11]K. J. Duda and D. R. Cheriton. Borrowed-Virtual-Time (BVT) scheduling: supporting latency-sensitive threads in a general-purpose scheduler. In Proceedings of the 17th ACM SIGOPS Symposium on Operating Systems Principles, volume 33(5) of ACM Operating Systems Review, pages 261.276, Kiawah Island Resort, SC, USA, Dec. 1999.

bvt scheduling
BVT scheduling
    • Main idea:
      • Virtual time.
    • Dispatching the runnable thread with the earliest effective virtual time (EVT).
  • The EVT for the thread is computed as:
    • the scheduler runs thread i if it has the minimum of all the runnable threads.
  • Parameter:
bvt scheduling1
BVT scheduling
  • Context switch decision:
  • Weight-fair sharing
    • (note: k*mcu=thread i running for t ms)
  • Parameter:
    • mcu = minimum charging unit
    • C = context switch allowance
bvt scheduling2
BVT scheduling

Virtual Time(Ei)



Real Time

bvt scheduling3
BVT scheduling
  • Sleeping adjustment:
    • Scheduler virtual time(SVT)
      • Scheduler variable indicating the minimum of any runnable thread.

Virtual Time(Ei)

gcc sleep for 15 real time unit, when gcc wakes up, is brought to SVT

Real Time

low latency dispatch
Low latency dispatch
  • Recall
    • can be set directly by a system call
    • Larger warp values provide lower latency dispatch than smaller values.
  • Another parameter:
low latency dispatch1
Low latency dispatch

Virtual Time(Ei)

  • mpeg run first because it is warped back 50 virtual units

Mpeg wake on t=5 and 15, and execute for 2.5 time.

Real Time

low latency dispatch2
Low latency dispatch

Virtual Time(Ei)

Li exceeded

Real Time

virtual address translation
Virtual address translation
  • Indeed, Xen need only be involved in page table updates.
    • Prevent guest OSes from making unacceptable changes.
  • Approach:
    • Xenregister guest OS page tables directly with the Memory Management Unit(MMU) and restrict guest OSes to read-only access.
    • Page table updates are passed to Xenvia a hypercall
  • Each VM has one or more Virtual network interfaces (VIFs).
    • VIFs are attached to a virtual firewall-router(VFR)
    • Domain0 is responsible for inserting and removing rules on VFR.
  • A VIF contains:
    • two I/O rings of buffer descriptors, one for transmit and one for receive.
    • Zero copy:
      • The guest OS exchanges an unused page frame for each packet it receives.
  • Fairness:
    • Xen implements a simple round-robin packet scheduler.
  • Only Domain0 has direct unchecked access to physical (IDE and SCSI) disks.
    • VM access persistent storage through the abstraction of virtual block devices (VBDs).
      • I/O ring mechanism.
    • A translation table is maintained within the hypervisor for each VBD.
      • Mapping VBD identifier and offset to the corresponding sector address and physical device.
    • Xen services batches of requests from competing domains in a simple round-robin fashion
evaluation environment
Evaluation Environment
  • Hardware:
    • Dell 2650 dual processor2.4GHz Xeon server
    • 2GB RAM,
    • a Broadcom Tigon 3 Gigabit Ethernet NIC,
    • a single Hitachi DK32EJ 146GB 10k RPM SCSI disk
  • OS:
    • Linux version 2.4.21
evaluation relative performance
EvaluationRelative Performance
  • Compare a VM performance with “bare metal”
    • Bare metal: a pure Linux OS directly install on physical machine.
  • This paper presents Xen, an x86 virtual machine monitor
    • allows multiple commodity operating systems to share conventional hardware.
    • without sacrificing either performance or functionality.
      • As our experimental results shows.
  • Ongoing work:
    • Porting BSD and Windows XP kernels to operate over Xen.
  • Paravirtualization indeed has good performance.
  • However, Domain-0 may be the bottleneck
    • a lot of work need domain-0 to validate or execute
  • OS need modification in order to install on Xen’s VM.
introduction of domain 0
Introduction of Domain-0
  • Domain-0
    • A special privilegeddomain(VM)
    • Serves as an administrative interface to Xen
    • The first domain launched when the system is booted
  • Note:
    • Domain-0(Dom0) = Privileged domain
    • Domain-U(DomU) = Unprivileged domain
a simple xen architecture
A simple Xenarchitecture




Direct physical access to all hardware

Dom0 exports the simplified generic class devices to each DomU

Configuration and monitoring Interface