1. Constructing Services with Interposable Virtual Hardware Author: Andrew Whitaker, Richard S. Cox, Marianne Shaw, and Steven D. Gribble Presenter: Huajing Li

2. Outline  Study Motivation  Overview of a previous VMM, Denali  An extensible VMM: μDenali  Key feature of μDenali: event routing framework  Application studies  Discussion

3. Study Motivation  Resurgence in popularity of VMMs A powerful platform for new system services Simplify problem solution Novel applications Applied in relevant study areas  A promising application of VMM: as a virtual machine service platform.

4. Design Limitation of Current VMMs  Little support was given for developing and deploying virtual machine services Non-programmable Non-extensible Self-tailored blackbox system without standard interfaces  Similar with many previous scenarios No standards or commonly-aware protocols are agreed Inter-operability is a serious issue Unnecessary efforts are devoted in repeated works

5. Interoperability Requirements  Goal: give programmers efficient facilities to develop services that manipulate the virtual machine interface.  What functionality should VMM provide to VM services? Interpose events (messaging across VMs) Extend hardware (resources) More?  How to support the previous two key services? Intuitively, as the solution we developed for distributed systems, we need to create a sub- system to coordinate between parts of the system.  Messaging protocol and supportive network  Resource assignment and management

6. Overview of a previous VMM, Denali  The novel VMM proposed in the paper is based on a previous VMM: Denali. Type-I VMM Design goal: support a large scale of VMs  Strategically modifies the virtual architecture (interrupt processing, handling non- virtualizable instruction, and timers) Support NetBSD OS

7. Denali Architecture  Virtual devices bridge between physical devices and VM usable interfaces. Virtual CPU Virtual MMU Virtual timers Virtual network Virtual disk

8. An extensible VMM: μDenali  Denali neither supports interposition nor extension.  μDenali is an updated version of Denali, in which three basic functions of a VMM are handled in a clean separated manner: Physical resource management Device namespace virtualization Virtual hardware event trapping and routing  Recall the two interoperability requirements discussed above. The separation of the three functions can perfectly meet them.

9. Event Routing Infrastructure in μDenali  Events (typed messages) are associated with destination ports.  The port mapping is configurable, managed by VMM.  A hardware of a VM (child) can be bound to another VM (parent) so that the parent VM has the privilege to interpose the child’s device.

10. μDenali Architecture

11. The NetBSD Interposition Library  Describe a set of operations that a parent VM can perform on a child. A protocol which consists of a set of downcalls sent to children VMs as well as a set of responses returned by children. Security concern: global events are not included in the protocol. The set of operations  Virtual machine control  I/O device interposition  Exposing μDenali internal state  Tracking and logging non-deterministic events

12. Event Routing  A framework to receive, route and deliver typed messages encoded by VMs. An interval messaging network  Ports are created as protected communication channel between VMs. Each virtual device in each VM has a set of standardized ports associated with it. A link between a child VM’s port and its parent VM’s port establishes the message delivery connection.

13. μDenali Port Tables  A centralized control by VMM Question: alternative approach? Pros and Cons  μDenali VMM maintains a table of port capabilities on behalf of each VM. Port capability includes receive, send and send-once rights. Stated in the paper: “From the point of view of μDenali, a virtual machine is simply a port table”. It is the parent VM’s responsibility to initialize the port table of a child VM.

14. μDenali Port Queues and Message Buffers  Messages are not stored by VMM Simplify the message delivery process. VMM is not involved in the checkpoints or recovery of a specific VM.  Each virtual device which owns message receiving port implements its own port queue. μDenali asks each VM to maintain ring buffers to store messages.

15. Example Port Table and Buffers

16. Applications  As use studies of μDenali Internet suspend / resume Drop-in network services Continuous rejuvenation Disk and swap device extensions  Supported by the interposition library provided by μDenali, the above services are easy to be implemented.

17. Discussion  Summary: this paper addresses the problem of lack of support for developing cooperative virtual machine services in current popular VMMs and proposes μDenali to solve it.  Question: What is the desired level of interoperability and extensibility? μDenali supports VMM-managed (centralized) resource and event sharing.

18. Thank you!