aiberia / persistent-evdev

Theory of Operation

Disconnecting evdev devices, such as with USB hotplug, can be problematic when using QEMU+Libvirt since it will not reopen the device when the device reconnects. Furthermore, existing libvirt hotplug mechanisms don't handle qemu:commandline options or cgroup dev permissions well.

A workaround is to create a udev proxy device and pass that virtual device to QEMU instead. This script works by detecting input device hotplug, automatically opening the device, and proxying the key/mouse events to the virtual udev device.

The capabilities of each physical device is cached so that the virtual udev devices can be created appropriately even when the real devices don't exist.

Setup

Requirements:

• Python 3
• Python evdev module: Arch: python-evdev, Fedora: python3-evdev, Gentoo: dev-python/python-evdev
• Python pyudev module: Arch: python-pyudev, Fedora: python3-pyudev, Gentoo: dev-python/pyudev

Important note: The python intepreter shebang in the persistent-evdev.py might need to be adjusted per your distro.

git clone https://github.com/aiberia/persistent-evdev.git /opt/persistent-evdev
ln -s /opt/persistent-evdev/systemd/persistent-evdev.service /etc/systemd/system/
ln -s /opt/persistent-evdev/udev/60-persistent-input-uinput.rules /etc/udev/rules.d/

(Optional) Selinux policy for Fedora:

make -C /opt/persistent-evdev/selinux

Edit the config file (/opt/persistent-evdev/etc/config.json) to point to your actual devices:

{
    "cache": "/opt/persistent-evdev/cache",
    "devices": {
        "persist-mouse0": "/dev/input/by-id/usb-Logitech_G403_Prodigy_Gaming_Mouse_078738533531-event-if01",
        "persist-mouse1": "/dev/input/by-id/usb-Logitech_G403_Prodigy_Gaming_Mouse_078738533531-event-mouse",
        "persist-mouse2": "/dev/input/by-id/usb-Logitech_G403_Prodigy_Gaming_Mouse_078738533531-if01-event-kbd",
        "persist-keyboard0": "/dev/input/by-id/usb-Microsoft_Natural®_Ergonomic_Keyboard_4000-event-kbd",
        "persist-keyboard1": "/dev/input/by-id/usb-Microsoft_Natural®_Ergonomic_Keyboard_4000-if01-event-kbd"
    }
}

Restart udev for the new rules, then enable and start the persistent-evdev service:

systemctl restart systemd-udevd
systemctl enable --now persistent-evdev.service

If everything worked new devices should be created as such:

$ ls -al /dev/input/by-id/
total 0
drwxr-xr-x 2 root root 140 Aug 17 02:23 .
drwxr-xr-x 4 root root 280 Aug 18 16:15 ..
lrwxrwxrwx 1 root root  11 Aug 17 02:23 uinput-persist-keyboard0 -> ../event256
lrwxrwxrwx 1 root root  11 Aug 17 02:23 uinput-persist-keyboard1 -> ../event257
lrwxrwxrwx 1 root root  10 Aug 17 02:23 uinput-persist-mouse0 -> ../event29
lrwxrwxrwx 1 root root  10 Aug 17 02:23 uinput-persist-mouse1 -> ../event30
lrwxrwxrwx 1 root root  10 Aug 17 02:23 uinput-persist-mouse2 -> ../event31

Example usage with Libvirt

Add the virtual devices to cgroup_device_acl in /etc/libvirt/qemu.conf as such:

cgroup_device_acl = [
    "/dev/null", "/dev/full", "/dev/zero",
    "/dev/random", "/dev/urandom",
    "/dev/ptmx", "/dev/kvm",
    "/dev/input/by-id/uinput-persist-keyboard0",
    "/dev/input/by-id/uinput-persist-keyboard1",
    "/dev/input/by-id/uinput-persist-mouse0",
    "/dev/input/by-id/uinput-persist-mouse1",
    "/dev/input/by-id/uinput-persist-mouse2"
]

Add the virtual devices to your VM XML:

<qemu:commandline>
    <qemu:arg value='-object'/>
    <qemu:arg value='input-linux,id=input1,evdev=/dev/input/by-id/uinput-persist-keyboard0,grab_all=on,repeat=on'/>
    <qemu:arg value='-object'/>
    <qemu:arg value='input-linux,id=input2,evdev=/dev/input/by-id/uinput-persist-keyboard1'/>
    <qemu:arg value='-object'/>
    <qemu:arg value='input-linux,id=input3,evdev=/dev/input/by-id/uinput-persist-mouse0'/>
    <qemu:arg value='-object'/>
    <qemu:arg value='input-linux,id=input4,evdev=/dev/input/by-id/uinput-persist-mouse1'/>
    <qemu:arg value='-object'/>
    <qemu:arg value='input-linux,id=input5,evdev=/dev/input/by-id/uinput-persist-mouse2'/>
</qemu:commandline>

See the usual Libvirt Evdev guides for more details.