Building a Linux VM image for ARMv8

February 24 2020 by Sebastian Sumpf

This brief article summarizes the steps necessary to build a runnable Linux kernel and initrd disk image that can be executed on top of Genode's custom VMM for i.MX8 platforms.

Prerequisites

• Download and install the Genode tool chain

• In order build an initial ram disk (i.e., initrd) a cross-compile tool chain for ARMv8 is required. We suggest a pre-build tool chain from Bootlin featuring the uClibc, for example, aarch64–uclibc–stable-2018.11-1.tar.bz2

• Install the Open Firmware device tree compiler

Building the Linux kernel

1. Get and extract the sources

    wget https://cdn.kernel.org/pub/linux/kernel/v4.x/linux-4.19.53.tar.xz
   
    tar -xJF linux-4.19.53.tar.xz
    cd linux-4.19.53
   

2. Configure the kernel

    make O=../build-linux-aarch64 ARCH=arm64 CROSS_COMPILE=/usr/local/genode-gcc/bin/genode-aarch64- defconfig
   

3. Build the kernel

    make O=../build-linux-aarch64 ARCH=arm64 CROSS_COMPILE=/usr/local/genode-gcc/bin/genode-aarch64-
   

4. The kernel image can is located at build-linux-aarch64/arch/arm64/boot/Image

Building an initrd using BusyBox

1. Download and extract BusyBox

    wget https://busybox.net/downloads/busybox-1.29.3.tar.bz2
   
    tar xjf busybox-1.29.3.tar.bz2
    mkdir build-busybox-aarch64
    cd busybox-1.29.3
   

2. Configure BusyBox

    make O=../build-busybox-aarch64 defconfig
    make O=../build-busybox-aarch64 menuconfig
   
    # check this setting in the menu
    [*] Setting ->  Build static binary (no shared libs)
   

3. Compile BusyBox with the installed cross-compile tool chain

    cd ../build-busybox-aarch64
    make CROSS_COMPILE=<path-to-tool-chain>/aarch64--uclibc--stable-2018.11-1/bin/aarch64-linux- install -j6
   

4. Useful setups for the ram disk

    cd <busbox-build-dir>/_install
   
   

   • create fstab

    <editor> etc/fstab
    none /proc  proc   defaults 0 0
    none /sys   sysfs  defaults 0 0
   

   • DNS resolution

    echo "nameserver 8.8.8.8" > etc/resolv.conf
   

   • Run level script using mdev

    <editor> etc/init.d/rcS
   
    #/bin/sh
   
    mkdir -p /proc
    mkdir -p /sys
    mkdir -p /var/shm
    mkdir -p /var/run
    mkdir -p /var/tmp
    mkdir -p /tmp
    mkdir -p /home/tc
    mkdir -p /root
    mkdir -p /dev
   
    /bin/mount -t tmpfs mdev /dev
    mkdir /dev/pts
    /bin/mount -t devpts devpts /dev/pts
   
    echo "Mount everything ${1}"
    /bin/mount -a
   
    echo /sbin/mdev > /proc/sys/kernel/hotplug
    /sbin/mdev -s
    echo "Finished"
    /bin/sh
   

5. Create the initial ram disk using cpio

    find . | cpio -H newc -o | gzip > ../initrd
   

6. Adjust the device tree information of Genode's VMM

    <editor> genode/repos/os/src/server/vmm/spec/arm_v8/virt.dts
   

7. Look for the chosen entry

   chosen {
     bootargs = "rdinit=/bin/sh";
     linux,initrd-start = <0x42000000>;
     linux,initrd-end   = <0x42113b86>;
     stdout-path = "/pl011@9000000";
   };
   

8. Adjust bootargs to use init

    bootargs = "init=/sbin/init";
   

9. Adjust the size of the ram disk

    ls -l initrd
   
    -rw-r--r-- 1 user user 697657 Jan 29 17:03 initrd
   
   

10. Convert size to Hex (0xaa539) and adjust initrd-end accordingly

     linux,initrd-end = <0x420aa539>;
    

11. Compile the device tree

     dtc <Genode-src>/repos/os/src/server/vmm/spec/arm_v8/virt.dts > <Genode-build-directory>/arm_v8a/bin/dtc
    

Have fun!