Hyper-V BIOS

OpenVMM currently relies on proprietary Hyper-V "PCAT"¹ BIOS firmware blobs in order to support booting and running various legacy x86 operating systems.

In the future, it would be great if OpenVMM could support alternative, open-source x86 BIOS firmwares, such as SeaBIOS.

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 BIOS:

• Pre-boot: the VMM's BIOS firmware loader writes the PCAT BIOS into guest RAM, and sets up the initial register state such that the VM will begin executing the firmware.

• At runtime: the BIOS code inside the VM communicates with a bespoke firmware_pcat virtual device, which it uses to fetch information about the VM's current topology, and to implement certain BIOS services (such as boot logging, efficient spin-looping, etc).

Acquiring the Hyper-V BIOS Firmware

Unfortunately, due to licensing restrictions, the OpenVMM project is not able to redistribute copies of the proprietary Hyper-V BIOS firmware blob.

That being said - Windows 11 ships copies of the PCAT BIOS firmware in-box under System32 as either vmfirmwarepcat.dll or vmfirmware.dll. When run on Windows / WSL2, OpenVMM will automatically scan for these files, and use them if present.