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This is related to my question on Ask Ubuntu: Add physical partition to QEMU/KVM virtual machine in virt-manager (see "Updates" near the end, if needed).

Is there a nice way to run any arbitrary script on the host every time a VM is started from virt-manager?

The reason is that I want to automagically unmount the ESP partition /dev/sda1 (mounted in the host as /boot/efi), and remount it when the VM is powered off.

[update] The script I was using can be found here.

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From libvirt: Hooks for specific system management

At present, there are five hook scripts that can be called:

  • /etc/libvirt/hooks/daemon
    Executed when the libvirt daemon is started, stopped, or reloads its configuration

  • /etc/libvirt/hooks/qemu
    Executed when a QEMU guest is started, stopped, or migrated

(...)

Script arguments

The hook scripts are called with specific command line arguments, depending upon the script, and the operation being performed.

(...)

The command line arguments take this approach:

  1. The first argument is the name of the object involved in the operation, or '-' if there is none.
    For example, the name of a guest being started.

  2. The second argument is the name of the operation being performed.
    For example, "start" if a guest is being started.

(...)

/etc/libvirt/hooks/qemu

  • Before a QEMU guest is started, the qemu hook script is called in three locations; if any location fails, the guest is not started. The first location, since 0.9.0, is before libvirt performs any resource labeling, and the hook can allocate resources not managed by libvirt such as DRBD or missing bridges. This is called as:
    /etc/libvirt/hooks/qemu guest_name prepare begin -

    The second location, available Since 0.8.0, occurs after libvirt has finished labeling all resources, but has not yet started the guest, called as:
    /etc/libvirt/hooks/qemu guest_name start begin -

    The third location, 0.9.13, occurs after the QEMU process has successfully started up:
    /etc/libvirt/hooks/qemu guest_name started begin -

(...)


So, in practice, this:

/etc/libvirt/hooks/qemu (without extension)

#!/bin/bash

if [[ $1 == "<domain_name>" ]] && [[ $2 == "start" ]] || [[ $2 == "stopped" ]]
then
  if [[ $2 == "start" ]]
  then
    # unmount logic here
  else
    # mount logic here
  fi
fi

Of course, edit <domain name>, implement the mount/unmount logic, and then restart libvirtd.service.

I'm making my full solution available at https://gitlab.com/ranolfi/rvirtesp.


An addendum: for this particular use case, you may find it useful to set up your fstab file with the option noauto and maybe also x-systemd.automount and x-systemd.device-timeout=<seconds>. See http://man7.org/linux/man-pages/man5/fstab.5.html and https://www.freedesktop.org/software/systemd/man/systemd.automount.html.

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