Introduction

OpenVMM is a modular, cross-platform Virtual Machine Monitor (VMM), written in Rust.

Although it can function as a traditional VMM, OpenVMM's development is currently focused on its role in the OpenHCL paravisor.

The project is open-source, MIT Licensed, and developed publicly at microsoft/openvmm on GitHub.

Cross-Platform

OpenVMM supports a variety of host operating systems, architectures, and virtualization backends:

Running in the OpenHCL paravisor

OpenVMM is the VMM that runs in the OpenHCL paravisor.

Unlike in traditional virtualization, where a VMM runs in a privileged host/root partition and provides virtualization services to a unprivileged guest partition, the "paravisor" model enables a VMM to provide virtualization services from within the guest partition itself.

It can be considered a form of "virtual firmware", running at a higher privilege level than the primary guest OS.

Paravisors are quite exciting, as they enable a wide variety of useful and novel virtualization scenarios! For example: at Microsoft, OpenHCL plays a key role in enabling several important Azure scenarios:

• Enabling existing workloads to seamlessly leverage Azure Boost (Azure's next-generation hardware accelerator), without requiring any modifications to the guest VM image.

• Enabling existing guest operating systems to run inside Confidential VMs.

• Powering Trusted Launch VMs - VMs that support Secure Boot, and include a vTPM.

Standalone VMM

OpenVMM can also run as a general-purpose VMM on a Windows, Linux, or macOS host. At the moment, this is primarily a development vehicle: most of the same code runs in OpenVMM on a host and OpenVMM in a paravisor, and it is often easier to test it on a host.

We will continue to build and test OpenVMM in this configuration, but currently we are not focused on the goal of supporting this for production workloads. It is missing many of the features and interface stability that are required for general-purpose use. We recommend you consider other Rust-based VMMs such as Cloud Hypervisor for such use cases.

Relationship to other Rust-based VMMs

OpenVMM's core security principles are aligned with those of the Rust-based Cloud Hypervisor, Firecracker, and crosvm projects, which is why we also chose to write OpenVMM in Rust. However, OpenVMM's unique goal of running efficiently in a paravisor environment made it difficult to leverage existing projects. OpenVMM requires fine-grained control over thread and task scheduling in order to avoid introducing jitter and other performance issues into guest VMs. It is difficult to achieve these requirements with traditional, thread-based designs.

Instead, OpenVMM uses Rust's async support throughout its codebase, decoupling the policy details of where code runs (which OS threads) from the mechanism of what runs (device-specific emulators). In a paravisor or resource-constrained environment, OpenVMM can run with one thread per guest CPU and ensure that device work is cooperatively scheduled along with the guest OS. In more traditional virtualization host, OpenVMM can run with one thread per device to use host CPUs to fully parallelize guest CPU and IO processing.

This approach has a significant impact on the design and implementation of the codebase, and bringing this model to an existing VMM would be a major undertaking. We came to the conclusion that a new project was the best way to achieve this goal.

We are indebted to the Rust VMM community for their trailblazing work. Now that the OpenVMM project is open source, we hope to find ways to collaborate on shared code while maintaining the benefits of the OpenVMM architecture.

Guest Compatibility

Similar to other general-purpose VMMs (such as Hyper-V, QEMU, VirtualBox), OpenVMM is able to host a wide variety of both modern and legacy guest operating systems on-top of its flexible virtual hardware platform.

• Modern operating systems can boot via UEFI, and interface with a wide selection of paravirtualized devices for services like networking, storage, and graphics.

• Legacy x86 operating systems can boot via BIOS, and are presented with a PC-compatible emulated device platform which includes legacy hardware such as IDE hard-disk/optical drives, floppy disk drives, and VGA graphics cards.

OpenVMM is regularly tested to ensure compatibility with popular operating systems (such as Windows, Linux, and FreeBSD), and strives to maintain reasonable compatibility with other, more niche/legacy operating systems as well.

To learn more about different facets of the OpenVMM project, check out the following links: