Twohypervisor

Twohypervisor is a coined term describing a virtualization architecture that stacks two hypervisors to provide layered isolation and control over workloads. In a typical deployment, an outer, hardware-facing hypervisor runs on the physical server and manages the underlying resources. A second hypervisor runs within a trusted partition created by the outer layer and hosts its own set of virtual machines. The approach aims to combine the strong isolation of a bare-metal hypervisor with the flexibility of an additional virtualization layer.

Operation relies on nested virtualization capabilities, with the outer hypervisor presenting virtual hardware to the inner hypervisor and the inner hypervisor presenting virtual hardware to its guest VMs. Performance and resource management are more complex due to the extra indirection, and features such as memory virtualization, I/O virtualization, and GPU pass-through must be carefully configured. Hardware-assisted virtualization features, such as nested page tables or extended page tables, are typically required, along with platform support for nesting and secure boot or trusted execution environments to protect the hypervisor chain.

Security benefits include stronger separation of tenant and management planes and containment of vulnerabilities to a specific layer. Risks encompass a larger attack surface, potential compatibility gaps, and increased maintenance effort. Operational considerations include monitoring, patching, and ensuring policy consistency across both layers.

Common use cases include multi-tenant cloud environments needing strict isolation, security research labs, and development platforms that require testing nested virtualization scenarios. However, twohypervisor deployments incur overhead, demand careful hardware and software provisioning, and may complicate live migration and performance for I/O- and GPU-intensive workloads.

Twohypervisor builds on established concepts of nested and layered virtualization. It is not a standard industry term, but the approach has been explored in research and by vendors as a means to improve isolation or testing capabilities. See also nested virtualization, Type 1 hypervisor, and Type 2 hypervisor.