#Breakdown

After a few days (years) of web browsing I still cannot understand why there is no pygrub alternative for qemu-kvm/libvirt. There are some workarounds around there, but all solutions have drawbacks (simplified objections):

1. Move your root filesytem to a disk with a parition table and install grub.
   ⬇ Too tangled to create/resize filesystem, a common task with VMs.
2. Create a small disk with /boot partition and grub[Gadi's Blog].
   ⬇ Requires to waste time (config, setup) and space (extra disk).
3. Use PvGrub.
   ⬇ I think this is an overhead for VMs (config) and I like to set kernel args in the domain configuration. I also known about this too late.
4. Use kexec hackery
   ⬇ It's not the best option for most people.
5. Surrender. Use host kernel or a selection of kernels stored some where.
   ⬇ Requires initrd with most modules, but some times, the module (like iptables modules) that you need is not there... regenerate initrd, reboot... I don't like it.

Pygrub is intended to be used with xen-tool to find /boot/grub/menu.lst and locate the kernel/initrd pair to be loaded. Also, in debian, exists the pv-grub-menu package which creates the /boot/grub/menu.lst. Clean and easy.

Libvirt allows the domain/os/bootloader[1][2] parameter to be set to pygrub but just works for Xen domains. I'd like to deepen into libvirt and allow kvm domains to use pygrub. However, I do not have enough time and the following workaround seems to be really functional.

#Solution The qemu script is a hook for libvirt. Works as a wrapper for pygrup:

• Parses the domain configuration on prepare/begin event,
• lets pygrub find the kernel through the available disks and
• copy the images to the REF_PATH/DOMAIN folder

#Installation (Based on Debian host/guest)

• Host (Dom0)

  • Copy qemu into /etc/libvirt/hooks/ and chmod 755
  • Install xen-utils-X.X to get pygrub (apt-get install --no-install-recommends xen-utils-4.1)
  • Install xmlstarlet and coreutils (basename/dirname) (apt-get install xmlstarlet coreutils)
  • Customize the script: set pygrub path (PYGRUB) and kernel images storage base path (BASE_PATH)
  • Restart libvirtd (service libvirt-bin restart or the equivalent for systemd)

• Guest (DomU)

  • Install linux-image-amd64 (or the one required for your VM)
  • If grub was installed, purge all grub packages.
  • Configure /boot/gurb/menu.lst
    • create a simple menu.lst with just "title, kernel and initrd" pointing to /vmlinuz /inird.img
    • or install pv-grub-menu, remove menu.lst and run update-menu-lst
  • If your are moving Xen VM to KVM review fstab, interfaces and persistent-net rules.
  • Configure Domain (note that DOMAIN is the exact libvirt domain name):
    • domain/os/kernel set to REF_PATH/DOMAIN/vmlinuz, ex: /srv/kvm/kernel-guest/test-vm/vmlinuz
    • domain/os/initrd set to REF_PATH/DOMAIN/initrd, ex: /srv/kvm/kernel-guest/test-vm/initrd
    • domain/os/cmdline the kernel command line. The command line set in menu.lst is ignored.