Xen and the Art of Virtualization

1. Xen and the Art of Virtualization Paul Barham, Boris Dragovic, Keir Fraser, Steven Hand, Tim Harris, Alex Ho, Rolf Neugebauer, Ian Pratt & Andrew Warfield Presented by Judy Fischbach

2. Overview • Motivations • How Xen works • Comparisons among Xen, hardware, and VMWare. • Xen and VMWare both designed for x86 architecture CS533 - Concepts of Operating Systems

3. Virtualization Challenges • Speed • Performance • Security • Resource Isolation • Functionality • Xen’s design goal CS533 - Concepts of Operating Systems

4. Types of virtualization • Two categories exist • Full Virtualization • Disco and VMWare • Paravirtualization • Xen CS533 - Concepts of Operating Systems

5. VMM Approach • In traditional VMM (Virtual Machine Monitor) the virtual hardware is functionally equivalent to the physical hardware • Possibly difficult to implement (x86 systems in particular) • Potentially useful to provide real and virtual resource information • The “guest” machine would not have access to this information CS533 - Concepts of Operating Systems

6. Xen Approach • Virtual machine is NOT 100% functionally equivalent to the hardware • Xen uses paravirtualization instead • Paravirtualization is a process where the guest operating system is modified to run in parallel with other modified systems • Advantage: Improved performance • Disadvantage: The hosted operating system must be modified before it can be hosted by the Xen Hypervisor (can be difficult) CS533 - Concepts of Operating Systems

7. Xen Design CS533 - Concepts of Operating Systems

8. Definitions • Guest Operating System • The operating system that Xen hosts • Domain • The virtual machine under which a guest operating system executes • Guest OS and a domain similar to idea of a program and a process • Hypervisor • A particular Xen version which handles low level functionality CS533 - Concepts of Operating Systems

9. Xen Implementation • What Xen does differently-- • CPU • Virtualization of the CPU • CPU Scheduling • Time & Timers • Memory Management • Virtual Address Translation • Physical Memory • Device I/O • Network • Disk • Control Transfer CS533 - Concepts of Operating Systems

10. CPU • Most change is required here by the guest OS • Xen takes exception to the basic idea that an OS is the most privileged • Privileged instructions • Instructions are paravirtualized by requiring to be validated/executed within Xen CS533 - Concepts of Operating Systems

11. CPU Virtualization • Virtualization easier with x86 • Why? • Has built in security levels (Rings 0, 1, 2, 3) • Ring 0 – OS Software (most privileged) • Ring 3 – User software • Ring 1 & 2 – Not used • Xen modifies the OS to execute on Ring 1 CS533 - Concepts of Operating Systems

12. CPU Scheduling and Timers • Borrowed Virtual Time algorithm is used in scheduling domains • Important to lessen problems of one domain executing code that might impact another domain. • Provides several different types of timers • Real Timer • Virtual Timer • Wall Clock Timer CS533 - Concepts of Operating Systems

13. Control Transfer • Exceptions and Events • Memory faults and software traps • Generally virtualized through Xen’s event handler • Two most frequent exceptions are: • System calls • Page faults • System calls and page faults are two examples of a ‘fast’ handler (one in which doesn’t use the hypervisor) CS533 - Concepts of Operating Systems

14. Paravirtualization of the MMU Paravirtualization Full Virtualization CS533 - Concepts of Operating Systems

15. Virtual Memory • When the guest OS requires a new page table, it allocates it from its own memory store • After allocation it becomes registered with Xen • OS then gives up all memory write privileges • All subsequent updates must be validated by Xen • Guest OS’s generally batch these update requests to amortize the cost of calling the hypervisor • Segmentation is virtualized likewise CS533 - Concepts of Operating Systems

16. Virtual Memory • Xen uses a design where • The guest OS’s themselves are responsible for allocation and managing their own hardware pages • Xen itself is located in an unused section at the top of address space to ensure that it is never paged out • This differs from the approach that Disco takes • Disco VMM goes a second level deeper in it’s indirection. • A VMM within VMM CS533 - Concepts of Operating Systems

17. Memory Management • Memory Management can be challenging • The key points • No software managed TLB exists in x86 • The TLB is not tagged which implies it must be flushed on a context switch CS533 - Concepts of Operating Systems

18. Device I/O Management • Data is transferred to and from domains through a buffer descriptor ring • Based on producer consumer pointers • One set used within guest OS • Another set used within Hypervisor • This allows separation between data and event notification CS533 - Concepts of Operating Systems

19. Hypervisor Control • In Xen • Domain0 is given greater access to the hardware and the hypervisor. It has: • A guest OS running above the domain • Hypervisor Manager software to manage elements within other existing domains. • In VMWare • Host OS acts underneath the domain. CS533 - Concepts of Operating Systems

20. Disk I/O Comparisons CS533 - Concepts of Operating Systems

21. New Domain Creation • Domain0 is privileged • New domains are created by Domain0 • Reduces the hypervisor complexity • Allows for a better debug environment CS533 - Concepts of Operating Systems

22. Networking • Networking in computers are important these days. • Because of this, Xen provides a “Virtual Firewall” • Domain0 creates firewall rules • Data transmission uses a buffer ring for outgoing • Data reception uses a different buffer ring for ingoing • Incoming data packets are analyzed according to Virtual Firewall rules • If any Virtual Firewall rules broken, packet is dropped CS533 - Concepts of Operating Systems

23. Other Hardware • Block diagram for Xen illustrates that the idea of Xen enabled hardware drivers still exists • VMWare operates similarly. • Two examples: • A sound card is installed, host machine will detect that you have a Sound Blaster enabled sound card. • A video driver is installed on guest OS to improve video performance (from VMWare). • Don’t forget, one more important part of virtualization. • Any other resources that the guest OS can access must be virtualized, not just the memory and cpu! CS533 - Concepts of Operating Systems

24. Citations • Diagrams from the following paper • http://i30www.ira.uka.de/teaching/coursedocuments/90/Xen.pdf CS533 - Concepts of Operating Systems