Arch Linux s390x Port with Systemd

This repository contains a complete, working s390x Arch Linux system with systemd support and full root filesystem that boots successfully on IBM mainframes and QEMU emulation.

Quick Start

# Build everything
make all

# Test initramfs-only system
make test

# Test with root filesystem
make test-rootfs

# Test with systemd as init
make test-systemd

What's Working

• Arch Linux Kernel: Linux 6.6.10 with Arch patches and CONFIG_UNIX/CONFIG_KMOD support (5.8MB)
• Initramfs: Complete 5.9MB initramfs with s390x busybox
• Boot Process: Successful boot to root filesystem with busybox shell
• Root Filesystem: COMPLETE! 150MB ext4 root filesystem with Arch Linux identification
• Systemd: WORKING! 42MB minimal systemd with mount/udevadm utilities - boots to "Welcome to Arch Linux!"
• Modern Init: Full systemd boot reaching Multi-User and Graphical targets
• QEMU Testing: Fully functional in s390x emulation
• Ready for Hardware: IPL configuration for real mainframes

Build Status

Core System Components

Component	Status	Details
Kernel	✅ Working	5.8MB bootable s390x kernel with CONFIG_UNIX/CONFIG_KMOD (6.6.10)
Initramfs	✅ Working	5.9MB with native s390x binaries
Boot Process	✅ Working	IPL → kernel → initramfs → root filesystem → systemd
Root Filesystem	✅ COMPLETE	150MB ext4 with working busybox switch_root
Systemd	✅ WORKING	42MB minimal systemd with mount/udevadm - reaches graphical target
mkinitcpio	✅ Fixed	Adapted for s390x architecture

Userspace Components

Component	Status	Details
Init System	✅ COMPLETE	Busybox in initramfs → switch_root → systemd
Shell	🟨 Minimal	Busybox sh (not bash)
Core Utilities	✅ Enhanced	Busybox applets + mount/udevadm/kmod utilities
Package Manager	❌ TODO	Pacman needs porting
Systemd Runtime	✅ WORKING	Full systemd boot with journal, udev, and basic services
Bash	❌ TODO	Needs s390x build
GNU Coreutils	❌ TODO	Replace busybox applets

Legend

• ✅ Complete - Fully working
• 🟨 Minimal - Working but minimal implementation
• ❌ TODO - Not yet implemented

Build System

Uses Fedora containers with Podman for cross-compilation:

# Container image:
s390x-archlinux-dev          # All-in-one development container

Build Targets:

• make all - Build complete system (kernel + initramfs + systemd)
• make kernel - Cross-compile Arch Linux kernel for s390x
• make initramfs - Generate initramfs with mkinitcpio
• make systemd - Build systemd natively on z/VM (requires .env configuration)
• make test - Test initramfs-only system with QEMU
• make test-rootfs - Test with root filesystem switching
• make test-systemd - Test with systemd as init (full modern boot)
• make clean - Clean build artifacts

Key Scripts:

• scripts/build-arch-kernel.sh - Cross-compile Arch Linux kernel
• scripts/build-initramfs-final.sh - Generate initramfs
• scripts/build-systemd-zvm.sh - Build systemd on z/VM
• scripts/deploy-and-build-systemd-zvm.sh - Orchestrate systemd build
• scripts/run-qemu-initramfs-only.sh - Test initramfs-only system
• scripts/test-qemu-rootfs.sh - Test with root filesystem
• scripts/test-qemu-systemd.sh - Test with systemd as init
• scripts/build-busybox-zvm.sh - Build static busybox on z/VM

z/VM Configuration

For building systemd and busybox natively on s390x hardware, you'll need access to an IBM z/VM system:

1. Copy the example configuration:

   cp .env.example .env
   

2. Edit .env with your z/VM details:

   ZVM_HOST=your.zvm.host
   ZVM_USER=your_username
   ZVM_SSH_KEY="-----BEGIN OPENSSH PRIVATE KEY-----
   ... your SSH private key content ...
   -----END OPENSSH PRIVATE KEY-----"
   

3. Build systemd:

   make systemd
   

The scripts will automatically load the .env configuration when needed.

Testing

# Install QEMU s390x
sudo pacman -S qemu-system-s390x  # Arch Linux
# or
sudo apt install qemu-system-s390x # Ubuntu/Debian

# Test initramfs-only system
make test

# Test root filesystem switching
make test-rootfs

# Test full systemd boot (requires systemd build)
make test-systemd

Expected Results:

• make test - System boots to emergency shell in initramfs-only mode (press Ctrl-A X to exit)
• make test-rootfs - Boots completely to root filesystem with busybox shell
• make test-systemd - Full systemd boot reaching "Welcome to Arch Linux!" and graphical target

Output Files

After building:

• boot/vmlinuz-linux - Ready-to-use s390x kernel
• boot/initramfs-linux.img - Working initramfs
• boot/rootfs-s390x.img - Root filesystem image (created by test-rootfs)
• boot/generic.ins - IPL configuration for real hardware
• boot/arch.prm - Kernel parameters
• boot/busybox-s390x-static - Static busybox binary built on z/VM (used by mkinitcpio)
• output/systemd-root/ - Complete systemd installation for root filesystem

Technical Details

s390x Adaptations:

• Big-endian architecture support
• IPL boot process (not BIOS/UEFI)
• Channel I/O subsystem drivers
• mkinitcpio modified for cross-architecture builds

Key Fixes Applied:

1. mkinitcpio adaptation - Fixed add_binary vs add_file issue for init scripts
2. Static busybox in initramfs - Built natively on z/VM for early boot operations
3. Architecture compatibility - Fixed base hook to exclude x86_64 binaries
4. switch_root support - Enables transition from initramfs to systemd on root filesystem
5. Kernel configuration - Added CONFIG_UNIX and CONFIG_KMOD for systemd support

System Boot Sequence

The complete boot process follows this sequence:

1. IPL (Initial Program Load) - IBM mainframe boot process
2. Kernel loads - Linux 6.6.10 s390x kernel with Arch patches
3. Initramfs unpacked - Contains busybox and init script
4. Early boot - Busybox provides utilities for mounting and setup
5. Root filesystem mounted - /dev/vda mounted to /new_root
6. switch_root - Busybox switch_root transitions to real root filesystem
7. Systemd starts - /usr/lib/systemd/systemd becomes PID 1