Introduction

For some time now with virt-install (developed under virt-manager) you have been able to specify a kernel and initial ramdisk to start a VM with. The only problem is that the VM will always start with that kernel/initrd (unless you change the definition manually). If you are rapidly testing operating system installations this can be problematic.

On the one hand, providing the kernel/initrd allows one to automate the install process from a Linux terminal, or even a script, by specifying the kernel/initrd and also the kernel command line options. However, it only gives us half the picture, because you’d then have to hand edit the libvirt definition of the machine to see if the installed machine was viable, OR you’d be lazy and just throw away the installed machine and assume it was good because the installation process finished without error; BAD.

When I was going through and testing the first iteration of the coreos-installer I resorted to creating my own .treeinfo file in order to workaround the shortcoming mentioned above. As a result of this I decided to file a feature request in virt-install to support this narrow, but useful use case of being able to specify a kernel/initrd and kernel arguments just for the install of a virtual machine. Cole Robinson and team were gracious enough to implement this feature request and now we have it in virt-manager 2.2.0 and later.

Let’s try it out!

Booting from Kernel/Initrd for Install

The way this was implemented is by utilizing sub argments to the --install option of virt-install. You can see the kernel, initrd and kernel_args* arguments from the help output:

$ virt-install --install help
--install options:
  clearxml
  bootdev
  initrd
  kernel
  kernel_args
  kernel_args_overwrite
  no_install
  os

Putting it all together we can call virt-install with either a local kernel/initrd or a remote kernel/initrd that can be accessed via HTTP(S). Here is an example that uses a remote kernel/initrd and does an install of Fedora CoreOS to the sda disk:

$ kernel=https://builds.coreos.fedoraproject.org/prod/streams/stable/builds/31.20200118.3.0/x86_64/fedora-coreos-31.20200118.3.0-live-kernel-x86_64
$ initrd=https://builds.coreos.fedoraproject.org/prod/streams/stable/builds/31.20200118.3.0/x86_64/fedora-coreos-31.20200118.3.0-live-initramfs.x86_64.img
$ kernel_args='ip=dhcp rd.neednet=1 console=tty0 console=ttyS0 coreos.inst.install_dev=/dev/sda coreos.inst.stream=stable coreos.inst.ignition_url=https://dustymabe.com/2020-01-30/auto-login-serial-console-ttyS0.ign'
$
$ virt-install --name fcos --ram 2048 --vcpus 2 --disk size=20 \
               --network bridge=virbr0 --graphics=none \
               --install kernel=${kernel},initrd=${initrd},kernel_args_overwrite=yes,kernel_args="${kernel_args}"

This will launch an instance into a serial console and you’ll see the install scroll by:

         Starting CoreOS Installer...
######################################################################## 100.0%
[   96.329276] coreos-installer-service[924]: coreos-installer install /dev/sda --ignition /tmp/coreos-installer-lPMZhg --firstboot-args rd.neednet=1 ip=dhcp  --stream stable
[   96.347656] coreos-installer-service[924]: Downloading stable image and signature
[  232.446299] coreos-installer-service[924]: gpg:                using RSA key 50CB390B3C3359C4
[  232.447695] coreos-installer-service[924]: gpg: Good signature from "Fedora (31) <fedora-31-primary@fedoraproject.org>" [ultimate]
[  233.824029] coreos-installer-service[924]: > Read disk 434.5 MiB/434.5 MiB (100%)
[  234.220963] coreos-installer-service[924]: Writing Ignition config
[  234.225558] coreos-installer-service[924]: Writing first-boot kernel arguments
[  234.264902] coreos-installer-service[924]: Install complete.

The installer then reboots the machine and the normal bootup will occur. After ignition is run, the real root is mounted and systemd brings the system up. Then the autologin happens on the serial console (because in this case we used auto-login-serial-console-ttyS0.ign).

[  OK  ] Started RPM-OSTree System Management Daemon.

Fedora CoreOS 31.20200118.3.0
Kernel 5.4.10-200.fc31.x86_64 on an x86_64 (ttyS0)

SSH host key: SHA256:AEg8Jt6J/zHz4iMMn25HoNFWOB4QWfSt3JaH2+xOqP8 (ECDSA)
SSH host key: SHA256:pELv/IoO4kYEtBMn4fjTq5sWShKyYAYSYgCVQGj93IU (ED25519)
SSH host key: SHA256:lp/ButDIqqkq3vrxsPMKslwtTFfaPg7XQF2qlEbywrc (RSA)
eth0: 192.168.122.9 fe80::5054:ff:feef:969e
localhost login: core (automatic login)

[core@localhost ~]$

And that’s a full test cycle! Hopefully this can be useful to anyone out there testing OS installs often.

Dusty