As reported by VGC, Microsoft updated its support website to reveal it has placed a temporary block on Windows 11 for users with those games installed.
“After installing Windows 11, version 24H2, you might encounter issues with some Ubisoft games,” Microsoft said. "These games might become unresponsive while starting, loading or during active gameplay.
"In some cases, users might receive a black screen. The affected games are Assassin’s Creed Valhalla, Assassin’s Creed Origins, Assassin’s Creed Odyssey, Star Wars Outlaws, and Avatar: Frontiers of Pandora.
Even after, some of it is pretty crazy.
Like the driver for controlling one vendor’s LED lights had a generic PCI FW updater (or something similar) included that it exposed to user space. This meant a) changing the LED colours or parameters required a firmware update rather than the firmware handling input from the system to adjust colours without new code, and b) other software could use this and just change the bus id of the target to update other firmware willy nilly.
It also had to compete for bus time and sending a full firmware update takes more time than a few colour update parameters. Average case might be ok, but it would make worst case scenarios worse, like OS wants to page in from disk 1 while a game needs to read shader code from disk 2 that it needs to immediately send to the GPU but the led controller decides it’s time to switch to the next theme in the list oh and there’s some packets that just came in over the network and the audio buffer is getting low. GPU ends up missing a frame deadline for the display engine and your screen goes black for a second while it re-establishes the connection between GPU and monitor.