Persistent attach of device with varying backend name

User Support
,
1

I am trying to persistently attach a device which has a varying backend name.

This is the guide that I followed to get the backend name

It returns something like “dm-85”, but it changes with each boot, so I cant do qvm-block attach -p vmname dom0:dm-85 since it won’t exist on next boot.

I can script it to attach it on boot, but then if I restart a VM it won’t be attached anymore.

What could be some solutions?

2

Could you use cron or systemd to periodically check if the VM is running, and if it is, to then attach the device?

3

Use libxl hook as I’ve suggested in another topic:

sudo mkdir /etc/libvirt/hooks/
cat << 'EOF' | sudo tee /etc/libvirt/hooks/libxl >/dev/null
#!/bin/bash
guest_name="$1"
libvirt_operation="$2"

if [ "$guest_name" = "yourqubename" ] && [ "$libvirt_operation" = "started" ]; then
    (
        exec 0</dev/null
        exec 1>/dev/null
        exec 2>/dev/null
        dev=$(basename $(readlink /dev/YOUR_LVM_VG/YOUR_LVM_IMAGE))
        if ! qvm-block attach "$guest_name" dom0:$dev; then
            qubesdb-write /qubes-block-devices/$dev/desc "YOUR_LVM_IMAGE"
            qvm-block attach "$guest_name" dom0:$dev
        fi
    ) & disown
fi
EOF
sudo chmod +x /etc/libvirt/hooks/libxl
3 Likes
4

Thank you, works like a charm

Some tips for future readers, to debug you can restart libvirtd manually instead of rebooting completely, systemctl restart libvirtd

exit 0 as quickly as possible if it’s not your target operation/VM

the subshell and redirection of output seems to prevent locking/hanging, if it’s not used the VM wouldn’t boot properly, probably for the same reason a terminal might hang until you hit cancel. And if that happens you can barely kill the VM because it’s in a locked state

look for alternative faster qvm- tools like qubesdb-read rather than qubes-block ls, any time wasted in the hook is time lost on boot

Some useful links here

https://wiki.archlinux.org/title/Libvirt#Hooks

My script is highly personalized with personal device names and VM names, so not useful for the public, but I hope this helps someone else solve the same issue :slight_smile:

1 Like
5

Thank you very much, this was exactly what I was looking for.
Maybe this can be merged into a community guideline.