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Xen

Xen. Bart Miller. Outline. Definition and goals Paravirtualization System Architecture The Virtual Machine Interface Memory Management CPU Device I/O Network, Disk Xen Timeline. Definition and Goals. An x86 Virtual Machine Monitor (aka Hypervisor) Developed in 2003

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Xen

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  1. Xen Bart Miller

  2. Outline • Definition and goals • Paravirtualization • System Architecture • The Virtual Machine Interface • Memory Management • CPU • Device I/O • Network, Disk • Xen Timeline

  3. Definition and Goals • An x86 Virtual Machine Monitor (aka Hypervisor) • Developed in 2003 • Approximately 60k lines of code • Goals: • 100 VMs per system • Support full multi-application OSes • No modifications to guest applications • Negligible sacrifice in performance • Fully isolate guests

  4. Paravirtualization • Full virtualization on x86 (ca. 2003) is complex and not efficient • Create a unique interface to the hardware • Let the Guest OS access the hardware directly when appropriate • Prevent the Guest OS from accessing functionality which could affect other guests or the VMM. • Must modify the Guest OS • For Linux, 2995 lines (1.36% of code base) • For Windows XP, 4620 lines (0.04% of code base)

  5. System Architecture

  6. Memory Management • Problem • x86 has a hardware managed TLB • Assumes single OS, does not support tagging nor managing in software • Context switch requires TLB flush • Solution • Guest OSes manage hardware page table • Direct read access; updates batched and validated by Xen • Xen resides in 64Mbyte section at the top of every address space

  7. CPU • Protection • x86 has 4 privilege levels, known as Rings. • Ring O is highest and Ring 3 lowest privilege • For Xen, the VMM executes in Ring O, the Guest OS executes in Ring 1, and the user programs execute in Ring 3. • Exceptions • The Guest OS registers a table of exception handlers with Xen • All are unmodified except the Page Fault handler, since it normally requires access to a privileged register (CR2)

  8. CPU (2) • System Calls • The Guest OS can register a “fast” exception handler • Executes without indirection • Xen verifies that the handler does not specify execution in Ring O • Interrupts • Interrupts are replaced by a lightweight event system • Asynchronous, relies on Ring buffer • Time • Guest OSes have access to “real” and “virtual” timers

  9. Device I/O • Network • VIF, VFR • Transmit and receive Ring I/O buffers • Domain O manages and enforces the firewall rules • Transmit: • A guest enqueues a request to the transmit ring • Xen validates the request against the firewall rules and forwards to the device • Receive: • A guest enqueues a receive request to the receive ring • Xen determines the appropriate recipient • The packet buffer is exchanged for a sacrificial page frame on the receiver’s ring

  10. Device I/O (2) • Disk access • Only Domain O can directly access physical disks • All DomUs communicate through Virtual Block Devices (VBD) • Channels are comprised of Ring buffers • Requests can be reordered by the Guest OS and Xen • Unless the Guest OS issues a reorder barrier

  11. Ring Structure

  12. Ring Structure (2)

  13. Xen Timeline • 2003: Initial release of Xen • 2005 was a significant year for Virtualization • Intel introduces VT-x, quickly utilized by Xen • Narrows performance gap between HVM and PVM • 2006: Amazon opens up public beta of EC2 • 2007: Live migration for HVM guests • 2008: PCI pass-through (VT-d) and ACPI S3 support • 2011: Xen support for DomO and DomU is added to the Linux kernel

  14. Questions?

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