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:
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Enabling existing workloads to seamlessly leverage Azure Boost (Azure's next-generation hardware accelerator), without requiring any modifications to the guest VM image.
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Enabling existing guest operating systems to run inside Confidential VMs.
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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.
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".