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Description

The MicroV Hypervisor is an open source, micro-hypervisor led by Assured Information Security, Inc., designed specifically to run micro VMs (i.e., tiny virtual machines that require little or no emulation).

Advantages:

Unlike existing hypervisors, MicroV's design has some unique advantages including:

• Cross-Platform Support: In open source, hypervisors that are capable of supporting guest virtual machines are mostly limited to Xen, KVM and VirtualBox. The first two only support Linux and BSD hosts and while VirtualBox expands this support to Windows and macOS, it sacrifices security. MicroV aims to support as many hosts as possible including Windows, Linux, BSD, macOS and any others without sacrificing performance and security. Today, MicroV already has support for Windows and Linux.

• Disaggregation and Deprivilege: None of the above mentioned hypervisors have a true focus on a reduced Trusted Computing Base (TCB). Xen comes the closest and has made amazing progress in this direction, but a fully disaggregated and deprivileged host has yet to be fully realized or supported. MicroV's design starts with a micro-kernel architecture inside the hypervisor, running most of it's internal logic at lower privilege levels. On Intel, when virtualization is enabled, the CPU is divided into the host and the guest. The hypervisor runs in the host, while the operating system runs in the guest. Both the host and the guest have a Ring 0 and Ring 3 privilege level. Monolithic hypervisors like Xen and KVM run the entire hypervisor in Ring 0 of the host (and in Xen's case some of the hypervisor runs in Ring 0 of the guest as well). Typically we call this Ring -1. This picture is far more complicated when you include System Management Mode (SMM) which adds two more Ring 0s, so lets pretend they do not exist for now. The thing is, this is not the only way to construct a hypervisor. Like the operating system in the guest, the hypervisor can use both Ring 0 and Ring 3 in the host, keeping the Ring 0 component as small as possible, while running most of the logic in Ring 3. In some ways, this is how KVM works. The kernel part of the hypervisor in KVM is the entire Linux kernel while VM management and emulation is handled in Ring 3 by QEMU (usually). The obvious problem with this design, besides the lack of cross-platform support is the size of the TCB is huge.

  In addition, with KVM, the physical devices are all managed from the host using the Linux kernel's device drivers. This is one way in which Xen is more deprivileged than KVM. Unlike KVM which runs device drivers in Ring 0 of the host, Xen runs device drivers in Ring 0 of the guest (specifically in Dom 0). MicroV aims to take a similar approach to Xen, keeping
  the code in the host as small as possible, and instead, delegating the guest operating system to manage the physical devices it is given.

• Performance: Another important focus of the project is on performance. Existing hypervisors make heavy use of emulation where virtualization could be used instead, Furthermore, Xen and KVM only support Linux and BSD hosts and therefore are not capable of leveraging some of the performance benefits of macOS and Windows such as power management. Other approaches to micro-kernel hypervisors attempt to provide their own device drivers, schedulers and power management algorithms. This limits their ability to widely support hardware, and guarantees a compromised user experience. The operating system that was built for a device should be the operating system that manages that device when possible.

• Scheduling: Although closely related to performance, MicroV leverages a hybrid design, incorporating the design goals of Xen to provide disaggregation and deprivilege while leveraging the scheduling benefits of hypervisor designs like KVM and VirtualBox. Specifically MicroV doesn't include its own scheduler, relying on the host to schedule each VM along with the rest of the critical tasks it must perform. Not only does this reduce the over complexity and size of MicroV, but it allows MicroV to leverage the advanced schedulers already present in the host while simultaneously removing the contention between the host scheduler and the hypervisor scheduler, often seen in Xen.

• Early Boot and Late Launch Support: Xen, KVM and VirtualBox support early boot (i.e. the hypervisor starts first during boot) or late launch (meaning the operating system starts first, and then the hypervisor starts). None of these hypervisors support both. Early boot is critical in supporting a fully deprivileged host while late launch is easier to set up, configure and use. Late launch is also a lot easier on developers, preventing the need to reboot each time a line of code changes in the hypervisor itself. MicroV aims to support both early boot and late launch from inception, giving both users and developers as many options as possible.

• AUTOSAR Compliant Although Xen, KVM and VirtualBox provide various levels of testing, continuous integration and continuous deployment, MicroV aims to take this a step further, providing the highest levels of testing possible. In addition, MicroV was re-engineered from the ground up using the AUTOSAR coding guidelines (MicroV is our third iteration of this hypervisor project). This will not only improve reliability and security, but also enable the use of MicroV in critical system environments were high levels of testing are required such as medical, automotive and government.

• Licensing: Most of the hypervisors available today in open source leverage the GPL license making it difficult to incorporate their technologies in closed source commercial products. MicroV is licensed under MIT. Feel free to use it however you wish. All we ask is that if you find and fix something wrong with the open source code, that you work with us to upstream the fix.

Disadvantages:

No design is without its disadvantages:

• Limited Guest VM Support: As stated above, on Intel, the CPU is divided into the host and the guest. The hypervisor runs in the host, and the operating system runs in the guest. The main operating system, which Xen would call Dom 0 can be any operating system. Today, we currently support Windows and Linux, and we even have limited support for UEFI. From there, MicroV lets you create additional, very small guest virtual machines. Currently, MicroV only has support for enlightened operating systems running in these guest VMs. Because they are enligntened, we can keep their size really small, and in some cases, remove the need for emulation entirely, which is where the term MicroVM comes from (i.e., a really small VM that requires little to no emulation). This ultametly means MicroV supports Linux, unikernels and enlightened applications, with no support for more complicated operating systems like Windows and macOS. Support for these types of operating systems is possible, but thats a bridge we will cross in the future.

• VM DoS Attacks: Since the main operating system is responsible for scheduling micro VMs for execution, it is possible that an attack in this operating system could prevent the micro VMs from executing (i.e., DoS attack). For most applications, this type of attack is a non-issue as isolation is more important than reslience against DoS attacks. With that said, there is no reason why a micro VM could not be in charge of scheduling VMs with its own scheduling and power management software (just like it would be possible to run all of the toolstack software in a dedicated micro VM as well). Like Xen, MicroV is designed to ensure these facilities are not dependent on the main operating system. The upstream project simply defaults to this type of configuration as its the larger, more prevelant use case. And keep in mind that there is always a tradeoff. Although the upstream approach is vulnerable to DoS attacks, implementing your own scheduler and power management software is no easy task, and should be limited to specific use cases (unless performance and battery life is not important).

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Compilation Instructions

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Usage Instructions

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