Patches submitted by Microsoft for the next version of the Linux kernel, 5.12, add the ability to boot the OS as the root partition on its Hyper-V hypervisor.
In November Microsoft added a "userspace interface for creating and running guest virtual machines while running on the Microsoft hypervisor" that depends on root partition support.
Although Linux already runs well on Hyper-V, the architecture requires that the Microsoft hypervisor has "at least one parent, or root, partition, running Windows." This root partition has direct access to the hardware, and creates the child partitions that run the guest operating systems such as Windows or Linux. The root partition includes a memory manager, management APIs, and virtual devices for I/O, disk control, and more.
The Hyper-V root partition has direct access to the hardware. The hypervisor itself is a kernel running on Ring -1
Microsoft has not stated how it intends to use its complete Linux virtualization stack but there are a number of possibilities. Currently the "Azure hypervisor system is based on Windows Hyper-V," as this document explains, and there could be performance and/or security advantages in using Linux for the root partition.
Azure runs increasing amounts of Linux virtual machines. Exact figures have not been published but Linux VMs have made up more than half the workload at least since 2019, when this was confirmed by a Microsoft engineer who said "the Linux usage on our cloud has surpassed Windows."
Kubernetes is primarily a Linux platform and appears to be ousting the Windows-based Service Fabric for container deployments, even for Microsoft's own Teams infrastructure.
Hypervisor technology also plays a key role in Windows 10. It is used to run Windows Subsystem for Linux 2, and also for Hyper-V Isolation, used by Microsoft Defender Application Guard, an optional Windows 10 feature that runs applications including Office and the Edge browser in containers for security reasons.
Win32 containers also play a role in the delated Windows 10X operating system. Might some future version of Windows run on a Linux root partition? Although that seems unlikely, the advent of a complete Microsoft Linux virtualization stack would make such things possible. ®