This is the report about the second tutorial for the Linux Kernel development: Building and booting a custom Linux kernel for ARM.

1. Cloning the Linux tree

For building a custom kernel, we need to choose a Linux tree. In this tutorial, we chose the IIO subsystem tree.

We cloned this tree from the testing branch and added the ./iio folder as an env var in our activation script.

git clone git://git.kernel.org/pub/scm/linux/kernel/git/jic23/iio.git "${IIO_TREE}" --branch testing --single-branch --depth 10

2. Configuring the Linux kernel compilation

With the cloned repository, we added export ARCH=arm64 to our activate script and setup our module configuration for this architecture.

make defconfig
make olddefconfig
ls -alFh .config"

Although that’s how the tutorial suggested, I believe that make olddefconfig wouldn´t make much effect after a make defconfig, since they have a similar behavior and the only difference is that defconfig overwrites everything and olddefconfig only changes configuration of new modules.

Then we could start libvirt daemon and default network and start an instance with an attaching console.

sudo systemctl start libvirtd
sudo virsh net-start default
sudo virsh start --console arm64

We could see that the VM is running successfully.

After that, we could fetch the vm_mod_list from the previous tutorial and save it locally, also adding a export LSMOD="${IIO_TREE}/vm_mod_list" to the script.

sudo virsh net-dhcp-leases default
scp root@<VM-IP-address>:~/vm_mod_list "${IIO_TREE}"

With the saved list of modules, we could change our .config applying the configuration from the VM.

make localmodconfig

After that, we accessed the ncofig menu to add a custom name for our kernel.

make nconfig

3. Bulding a custom Linux kernel

To build this custom kernel, we needed the correct compiler for this archtecture, also adding export CROSS_COMPILE=aarch64-linux-gnu- to the script.

sudo pacman -Syy && sudo pacman -S aarch64-linux-gnu-gcc

With everything configured, we could, then, compile our kernel.

make -j$(nproc) Image.gz modules

This process took a lot of time!

4. Installing modules and booting the custom-built Linux kernel

After that, we just needed to install this modules into the VM and boot the custom kernel. We did that by adding export VM_MOUNT_POINT="${VM_DIR}/arm64_rootfs" and export INSTALL_MOD_PATH="$VM_MOUNT_POINT" to the script. Also, we mounted the VM and defined the image from our custom kernel as the kernel for the VM.

mkdir "$VM_MOUNT_POINT"
sudo guestmount --rw --add "${VM_DIR}/arm64_img.qcow2" --mount /dev/sda2 "${VM_MOUNT_POINT}"
sudo --preserve-env make -C "${IIO_TREE}" modules_install
sudo guestunmount "$VM_MOUNT_POINT"

Then we created a new VM from this configuration.

sudo virsh undefine arm64
create_vm_virsh

When runinng the following command in the VM, we could see that it was using the custom kernel.

uname --kernel-release