How Microsoft’s New Linux Patches Enable Nested Hyper‑V Virtualization

Microsoft recently released a series of patches for the Linux kernel that add support for running nested Microsoft Hyper‑V hypervisors.

The patch series aims to add support for running a nested Microsoft Hypervisor (hypervisor) where certain privileged hypercalls must be handled by the L0 Hypervisor (the physical Hyper‑V instance) instead of the L1 Hypervisor (the nested one). These patches detect such hypercalls and replace them with nested hypercalls.

The changes included in the patches are:

mshv: add support for detecting nested hypervisors

hv: set synic registers when running a nested root partition

hv: add an interface to execute nested hypercalls

hv: enable the vmbus driver for nested root partitions

hv, mshv: modify interrupt vectors for nested root partitions

Hyper‑V is Microsoft’s hardware virtualization product that allows the creation and operation of virtual machines, each acting as a full computer. Nested virtualization lets users run Hyper‑V inside a Hyper‑V virtual machine, offering performance improvements compared to running a hypervisor directly on bare metal by leveraging the L0 Hypervisor’s interfaces to optimize the L1 Hypervisor.

If these patches are merged, Linux users will be able to run multiple Windows instances with nested virtualization, a feature primarily aimed at enterprise scenarios.

The patch set adds just over 100 lines of new code, and if the code review proceeds smoothly, this nested Microsoft hypervisor support may be merged into the Linux 6.2 kernel cycle.

Related link: https://lore.kernel.org/lkml/[email protected]/