This is a collection of scripts and notes that I use for Linux kernel development.

• bin: scripts that I add to my $PATH
• configs: kernel configs
• modules: experimental/testing kernel modules
• notes: various development notes
• packages: Arch Linux packages used for development
• scripts: tests, bug reproducers, and other one-off scripts

I use QEMU for kernel development. My setup allows for running kernels straight off of the host system without installing onto the guest, which makes for a super fast edit-compile-test cycle.

vm.py is my VM management script. It fills a role similar to that of something like libvirt but is much simpler.

Each VM has its own directory under a top-level VM directory containing all of its disk images and its configuration file.

vm.py create creates a new VM under the top-level VM directory. A few basic configuration options (CPUs, memory, disk size) can be given. The new VM only has a virtio-net NIC, a (blank) virtio-blk root disk, and a serial console.

The configuration file for each VM is named config.py. It's a Python script which must define a qemu_options list and kernel_cmdline list. It may also define a qemu_arch string.

vm.py run runs a VM. Arbitrary QEMU options may be added to those in the configuration. The killer feature, however, is the -k option: this runs the VM with a kernel from the host machine.

vm.py archinstall installs and configures Arch Linux on a new VM. It automatically downloads the latest Arch Linux ISO and runs an automated setup process. If there is an error during this process, you will be dropped into a shell on the VM. If this happens, you can edit install.sh on the VM and try again manually (but please also open an issue on GitHub so I can fix it). If the installation process succeeds, the VM will power off, after which you can restart it with vm.py run. The default username and password are vmuser.

vm.py may be configured in ~/.config/vmpy.conf:

[Paths]
# Top-level VM directory. Defaults to "~/vms".
VMs=~/vms
# Directory to download installer ISOs to. Defaults to "iso" directory under
# the VMs directory.
ISOs=~/Downloads
# Directory to look in for `vm.py run -k`. Unset by default.
Builds=~/builds

Running a custom kernel build on a VM usually requires installing that kernel on the VM. This is wasteful and slow. Instead, I use a combination of QEMU's -kernel option and VirtFS to allow booting a kernel straight off of the host system.

QEMU's -kernel option boots the VM straight into a kernel image. This, however, doesn't handle kernel modules. For that, vm.py provides a VirtFS to the guest containing the modules, which the guest mounts while booting (see scripts/vm-modules-mounter.service). vm.py archinstall automatically installs vm-modules-mounter. You can install it manually by copying vm-modules-mounter.service to /etc/systemd/system and running systemctl enable vm-modules-mounter.service.

Note that this setup requires a few kernel configuration options; see configs/vmpy.fragment.

I keep my kernel build configuration files in the configs directory. Rather than keeping full, generated config files, which are noisy and hard to manage, I keep configuration "fragments" with only the configuration options I care about and use kconfig.py to merge them.

kconfig.py also augments configuration files with an include command that reads another configuration file and inserts it into the current file verbatim. For example, suppose we have the following files:

file1.config:

CONFIG_FOO=y
CONFIG_BAR=m

include "file3.config"

CONFIG_BAZ=y

file2.config:

CONFIG_QUX=m

file3.config:

CONFIG_FOO=m
CONFIG_BAZ=m
CONFIG_QUX=y

kconfig.py file1.config file2.config would produce a configuration file with CONFIG_FOO=m, CONFIG_BAR=m, CONFIG_BAZ=y, and CONFIG_QUX=m.

Included filenames are interpreted relative to the current file.

Finally, kconfig.py checks the generated configuration file to make sure that all options were set as desired (which may not be the case if some dependencies were not satisfied, for example).