UEFI: mu_msvm

OpenVMM currently uses the mu_msvm UEFI firmware package in order to support booting and running modern EFI-boot capable operating systems.

What about OVMF, or other firmware packages?

In the future, it would be useful to also support alternative UEFI firmware packages, such as OVMF.

Please reach out of if this is something you may be interested in helping out with!

Two OpenVMM components work in tandem in order to load and run the mu_msvm UEFI firmware:

  • Pre-boot: the VMM's UEFI firmware loader:

    1. Reads and validates the firmware's build and interface version information
    2. Writes the mu_msvm UEFI firmware package into guest RAM
    3. Writes VM topology information, and mu_msvm-specific config data into guest RAM
    4. Initializes register state such that the VM will begin executing from UEFI

  • At runtime: the UEFI code within the Guest interfaces with a bespoke firmware_uefi device in order to implement certain UEFI services, such as NVRam variable support, watchdog timers, logging, etc.

Firmware interface version

Newer mu_msvm images contain a machine-readable version record in the DXE firmware volume. OpenVMM locates its dedicated FFS file by GUID and logs the record's release version, source commit, build flags, structure version, and firmware interface version when loading the image.

OpenVMM accepts the supported interface major version and that major's minimum minor version or any newer minor version. A present record that is malformed or has an incompatible interface version prevents the VM from starting. Use --uefi force_firmware_version to log a warning and continue in that case.

Firmware published before this record was introduced remains usable. OpenVMM logs a warning when the dedicated FFS file is absent and continues loading the image.

Acquiring a copy of mu_msvm

The cargo xflowey restore-packages script will automatically pull down a precompiled copy of the mu_msvm UEFI firmware from the microsoft/mu_msvm GitHub repo.

Alternatively, for those that wish to manually download / build mu_msvm: follow the instructions over on the microsoft/mu_msvm repo, and ensure the package is extracted into the .packages/ directory in the same manner as the cargo xflowey restore-packages script.