On-Demand 8bitdo Controller Passthru to VMs in Linux

I am using a Windows VM for my gaming fix while having Linux as the host OS for my desktop. For gaming, I got a 8bitdo controller that can connect through both bluetooth and USB with a provided USB dongle. This is nice because I can use the bluetooth connection with Linux for native Linux gaming and switch to the USB connection for gaming on my Windows VM.

While it is very straightforward to passthru a connected host USB device to a VM in Linux using virt-* tools and UX, one annoying thing about this is if you (like me) will unplug the USB device from time to time. Also, the 8bitdo controller will turn off to conserve battery after a period of not being used. When this happens, the USB dongle also turns itself off. If either of these happens, the VM will not recognize the device is no longer there (if the VM is running) and even if you plug it back in, it will still not recognize the device is there. I fixed this with a shell script that attaches and detaches the passthru USB device on-demand. In addition, the USB passthru configuration on the VM was made optional so there’s no annoying error message if I try to start the VM without the device being attached.

Here’s the configuration for the USB device to make it optional during boot of the USB – to get the vendor and product id, run lsusb and find your device:

<hostdev mode="subsystem" type="usb" managed="yes">
  <source>
    <vendor id="0xdead"/>
    <product id="0xbeef"/>
    <address bus="1" device="22"/>
  </source>
  <alias name="hostdev4"/>
  <address type="usb" bus="0" port="2"/>
</hostdev>

And here’s the script that allows on-demand attach/detach of the passthru USB device:

#!/bin/bash

VM_NAME="my_vm_name"
VENDOR_ID="dead"  # Replace with your device's vendor ID
PRODUCT_ID="beef" # Replace with your device's product ID

# Function to check if the USB device is connected
check_device() {
    lsusb | grep -q "$VENDOR_ID:$PRODUCT_ID"
    return $?
}

# Function to attach the USB device to the VM
attach_device() {
    XML_CONFIG=$(cat <<EOF
<hostdev mode='subsystem' type='usb' managed='yes'>
  <source>
    <vendor id='0x$VENDOR_ID'/>
    <product id='0x$PRODUCT_ID'/>
  </source>
</hostdev>
EOF
    )
    echo "$XML_CONFIG" | virsh attach-device "$VM_NAME" /dev/stdin --current
}

# Function to detach the USB device from the VM
detach_device() {
    XML_CONFIG=$(cat <<EOF
<hostdev mode='subsystem' type='usb' managed='yes'>
  <source>
    <vendor id='0x$VENDOR_ID'/>
    <product id='0x$PRODUCT_ID'/>
  </source>
</hostdev>
EOF
    )
    echo "$XML_CONFIG" | virsh detach-device "$VM_NAME" /dev/stdin --current
}

# Main loop
if check_device; then
    if ! virsh dumpxml win11 | grep -A3 "<vendor id='0x$VENDOR_ID'/>" | grep -q "<product id='0x$PRODUCT_ID'/>"; then
        echo "USB device detected. Attaching to VM..."
        attach_device
    else
        echo "USB device already attached to VM..."
    fi
else
    if virsh dumpxml win11 | grep -A3 "<vendor id='0x$VENDOR_ID'/>" | grep -q "<product id='0x$PRODUCT_ID'/>"; then
        echo "USB device disconnected. Detaching from VM..."
        detach_device
    else
        echo "USB device already detached from VM..."
    fi
fi