With the fact that there are many different firmware implementations, we tried to decouple today's monolithic UEFI firmware binary to two independent components: bootloader and payload.

Basically, bootloader initializes the silicon hardware and payload prepares the OS required data and services. Bootloader passes control to payload.

https://universalpayload.github.io/documentation/spec/spec.html defines the universal interface between bootloader and payload. So that different bootloaders can work with different payloads, initializing different hardware and booting different OSes.
The interface document is in very draft phase. Any feedback is welcome.

We also developed the POC code to demonstrate the idea. Please use below steps to get the code that uses SBL as the bootloader and EDKII UEFI Payload as the payload. This POC is being developed for QEMU Q35 virtual machine.

  1.  Run "git clone https://github.com/universalpayload/tools.git payload"

This step downloads the initial tools that will setup the dev environment.

  1.  Run "py -3 clone_and_build_sbl_with_uefipayload.py" in the "payload" directory

This script downloads branched SBL (https://github.com/universalpayload/slimbootloader.git) and edk2 (https://github.com/universalpayload/edk2.git).

Then it builds the firmware binary "SlimBootloader.bin" in "codeworkspace" directory.

  1.  Boot QEMU by running "qemu-system-x86_64.exe -machine q35 -pflash  codeworkspace\SlimBootloader.bin  -serial file:test.log"

Because the code is under active development, please contact us when you cannot build or boot successfully.

Besides the SBL, we modified coreboot (https://github.com/universalpayload/coreboot.git) to let it conform to the universal interface as a bootloader.
Besides the EDK2 UEFI Payload, we created a payload (https://github.com/universalpayload/linuxpayload.git) that can boot Linux.

Thanks,
Ray