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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.

Footnotes

  1. Fun fact: the term "PCAT" refers to the venerable IBM Personal Computer AT, as a nod to this BIOS's early history as a fairly stock PC/AT compatible BIOS implementation. ↩