Nested virtualization

What is Nested Virtualization?

Nested Virtualization refers to the capability of running a hypervisor within another hypervisor, effectively allowing you to run virtual machines (VMs) inside of other VMs. This concept is particularly useful for scenarios where you need to test or simulate different environments that include virtualization themselves, such as cloud environments, development, or testing hypervisor-level features.

For example:

  • You have a physical server (host) running a hypervisor like VMware ESXi, KVM, or Hyper-V.
  • Inside this hypervisor, you create a virtual machine (VM).
  • Inside this VM, you can run another hypervisor, which can then run additional virtual machines.

Nested virtualization is useful for testing, development, and training purposes, where you need the full functionality of a hypervisor without needing dedicated physical hardware for each layer.

How Does Nested Virtualization Work?

Normally, a hypervisor virtualizes the underlying physical hardware and abstracts the CPU and memory resources for virtual machines. Nested virtualization extends this by allowing a virtual machine to behave as a host for other virtual machines. This is done by emulating or passing through certain CPU features that allow the guest hypervisor to access hardware-assisted virtualization features, like Intel VT-x or AMD-V.

In practice, this requires:

  1. CPU support: The CPU must support hardware-assisted virtualization features (e.g., Intel VT-x or AMD-V) and enable these features for nested environments.
  2. Hypervisor support: The primary hypervisor needs to support the nested virtualization feature and expose virtualization extensions to its guest VMs.

Levels of Nested Virtualization

The levels of nested virtualization refer to how deep you can nest hypervisors, creating VMs inside VMs recursively.

  1. Level 0 (L0): This is the physical machine with hardware. The hypervisor runs directly on this physical machine.
  2. Level 1 (L1): The hypervisor running directly on the physical hardware (L0) hosts virtual machines. These virtual machines run on the L1 hypervisor.
  3. Level 2 (L2): Inside an L1 VM, you can install another hypervisor. This nested hypervisor runs L2 virtual machines.
  4. Level 3 (L3) and Beyond: Theoretically, you can continue nesting hypervisors. Each new level adds a layer of abstraction where an L2 VM hosts a hypervisor, creating L3 VMs, and so on.

In practice, the number of nested levels you can achieve depends on several factors:

  • Hardware limitations: CPU and memory constraints can limit the practicality of multiple levels of nesting.
  • Hypervisor limitations: Not all hypervisors support deep nesting, and performance degrades significantly with more nested layers.
  • Performance: Each layer adds overhead, so while you may theoretically be able to nest virtualization to a deep level (e.g., L3, L4), it may not be practical due to significant performance losses.

Most common use cases typically involve L1 and L2 layers, which are practical for testing and development environments.

Example Use Cases for Nested Virtualization

  1. Cloud Service Providers: Nested virtualization allows cloud providers like Azure, AWS, and Google Cloud to offer virtual machines that can themselves act as hypervisors. This enables customers to run their own hypervisor within a cloud-hosted VM.
  2. Development and Testing: Developers can test hypervisor behavior without needing multiple physical servers. This is essential for debugging hypervisor issues or developing virtualization products.
  3. Training and Labs: Nested virtualization is used in training environments where users need to simulate a full hypervisor setup, for instance, in a VMware or Hyper-V lab.
  4. CI/CD Pipelines: Running CI/CD pipelines that require virtualized environments for testing can utilize nested virtualization to avoid requiring multiple physical machines.

Key Takeaways

  • Nested Virtualization: Running a hypervisor inside a virtual machine.
  • Levels: Theoretically infinite, but practically limited due to performance and hardware constraints.
  • Use Cases: Testing, cloud environments, training, and development.

This should give you a comprehensive understanding of nested virtualization and its practical applications. If you have any more questions or need further clarification, feel free to ask!