# OpenHCL

OpenHCL is an execution environment which runs OpenVMM as a **paravisor**.

Unlike in traditional virtualization, where a VMM runs in a privileged host/root
partition and provides virtualization services to a unprivileged guest
partition, the "paravisor" model enables a VMM to provide virtualization
services from _within_ the guest partition itself.

It can be considered a form of "virtual firmware", running at a higher privilege
level than the primary guest OS.

Paravisors are quite exciting, as they enable a wide variety of useful and novel
virtualization scenarios! For example: at Microsoft, OpenHCL plays a key role in
enabling several important Azure scenarios:

- Enabling existing workloads to seamlessly leverage [Azure Boost] (Azure's
  next-generation hardware accelerator), without requiring any modifications to
  the guest VM image.

- Enabling existing guest operating systems to run inside [Confidential VMs].

- Powering [Trusted Launch VMs] - VMs that support Secure Boot, and include a
  vTPM.

* * *

To learn more about OpenHCL's architecture, please refer to
[OpenHCL Architecture](../reference/architecture/openhcl.md).

```admonish note
_Note:_ As you explore the OpenVMM repo, you may find references to the term
**Underhill**.

Underhill was the former codename for OpenHCL, so whenever you see the term
"Underhill", you can treat it as synonymous to "OpenHCL".

We are actively migrating existing code and docs away from using the term
"Underhill".
```

[VSM]: https://learn.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/vsm
[Virtual Trust Levels]: https://learn.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/vsm
[Azure Boost]: https://learn.microsoft.com/en-us/azure/azure-boost/overview
[Confidential VMs]: https://azure.microsoft.com/en-us/solutions/confidential-compute
[Trusted Launch VMs]: https://learn.microsoft.com/en-us/azure/virtual-machines/trusted-launch
[TDX]: https://www.intel.com/content/www/us/en/developer/tools/trust-domain-extensions/overview.html
[SEV-SNP]: https://www.amd.com/content/dam/amd/en/documents/epyc-business-docs/white-papers/SEV-SNP-strengthening-vm-isolation-with-integrity-protection-and-more.pdf
