microvm.nix with USB passthrough

microvm-with-usb-passthrough.nix

# declarative microvms do not run as root. in order for USB passthrough to work they need access
# to the USB devices in /dev/bus/usb. qemu does not print any kind of error if it does not have
# access; the devices just don't show up. 
# 
# we can fix this by using udev to grant the "kvm" group access to the USB devices.

# in the guest:
microvm.devices = [
  # RTL2838UHIDIR
  # Realtek Semiconductor Corp. RTL2838 DVB-T
  { bus = "usb"; path = "vendorid=0x0bda,productid=0x2838"; }
  # Sonoff Zigbee 3.0 USB Dongle Plus
  # Silicon Labs CP210x UART Bridge
  { bus = "usb"; path = "vendorid=0x10c4,productid=0xea60"; }
];

# on the host:
services.udev.extraRules = ''
  # RTL2838UHIDIR
  # Realtek Semiconductor Corp. RTL2838 DVB-T
  SUBSYSTEM=="usb", ATTR{idVendor}=="0bda", ATTR{idProduct}=="2838", GROUP="kvm"

  # Sonoff Zigbee 3.0 USB Dongle Plus
  # Silicon Labs CP210x UART Bridge
  SUBSYSTEM=="usb", ATTR{idVendor}=="10c4", ATTR{idProduct}=="ea60", GROUP="kvm"
'';