To boot Linux, a kernel image (vmlinuz) and a suitable initial RAM disk (initrd.img) need to be created.
The Linux kernel for the primary VM can be built using the following command-line:
git clone https://github.com/torvalds/linux.git cd linux ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make defconfig ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make -j24
The compiled image is stored in arch/arm64/boot/Image. This will later be copied to the Hafnium RAM disk's root as vmlinuz.
From the Hafnium root directory, the following commands can be used to compile the kernel module, replacing <kernel-path> with the path to the kernel checked out in the previous section:
cd hafnium/driver/linux/ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- KERNEL_PATH=<kernel-path> make
The compiled module is called hafnium.ko, and will later be copied into the RAM disk for Linux.
To make Linux useful, it needs a shell. These following instructions will construct a file system for the Linux RAM disk with the BusyBox shell as the init process.
git clone git://busybox.net/busybox.git cd busybox ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make defconfig ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make menuconfig
At this point you should ensure that the option Settings > Build static binary (no shared libs) is selected. Then you can proceed with the following commands:
ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make -j24 ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make install cd _install mkdir proc mkdir sys mkdir -p etc/init.d cat <<EOF > etc/init.d/rcS #!bin/sh mount -t proc none /proc mount -t sysfs none /sys EOF chmod u+x etc/init.d/rcS grep -v tty ../examples/inittab > ./etc/inittab
At this point you can copy into the current directory additional files you may want in the RAM disk, for example, the kernel module built in the previous section. Assuming the BusyBox root directory is in the same parent directory as the Hafnium root directory:
cp ../../hafnium/driver/linux/hafnium.ko .
Then run the following commands:
find . | cpio -o -H newc | gzip > ../initrd.img cd ..
The resulting file is initrd.img. It should be copied to the Hafnium RAM disk's root.