UEFI is event-based, not thread-based. Earlier this month, Michael Zimmermann of the EFIDroid project posted a message on the EDK2-devel list about EFIDroid’s thread library support for UEFI, which is based on the Little Kernel threads implementation, and comparing it to the GreenThreads-UEFI project. Edited (footnotified) version of Michael’s message below.

IMO this [GreenThreads-UEFI] library[0] has some crucial problems like changing the TPL during context switching. For my project “EFIDroid” I’ve invested many months analyzing, testing and implementing my own threading implementation based on LK(LittleKernel, a MIT licensed project) threads and get/set -context. The result is a pretty stable implementation which can even be used in UEFI drivers[1]. I’m currently using this lib for my LKL(LinuxKernelLibrary) port to be able to use linux touchscreen drivers in UEFI – so you could say it has been well tested. The only “problem” is that it only supports ARM right now and that the get/set context implementation was copied (and simplified) from glibc which means that this part is GPL code.

From the Little Kernel web site:

Who is using LK?
* LK is the Android bootloader and is also used in Android Trusted Execution Environment – “Trusty TEE” Operating System.
* Newer Android phones have some chance of LK running all the time alongside Linux.
* A few ARM SoC manufacturers use LK as their default bootloader such as DragonBoard 410c based on Qualcomm Snapdragon 410 processor.
* The Fuchsia Operating System’s microkernel, Zircon is based on LK.

[0] https://github.com/Openwide-Ingenierie/GreenThreads-UEFI
[1] https://github.com/efidroid/uefi_edk2packages_EFIDroidLKLPkg/tree/master/UEFIThreads
http://efidroid.org/

https://github.com/littlekernel
https://github.com/littlekernel/lk/wiki/Introduction
https://github.com/littlekernel/lk/blob/master/kernel/thread.c

https://android.googlesource.com/kernel/lk/

Full message: 2017-11-02 post on EDK2-devel.