I noticed a #nixos test occasionally failing, and ended up tracing it down to a bug in the qemu USB EHCI code 🤓
Follow along at: https://github.com/NixOS/nixpkgs/issues/170803
Learned a lot about qemu and EHCI.
Fix now submitted upstream: https://lists.nongnu.org/archive/html/qemu-devel/2022-05/msg01484.html
Actually more likely in 7.1.0 which is expected in August
https://wiki.qemu.org/Planning/7.1