s390x-initramfs-braindump.txt

apt install -y crossbuild-essential-s390x

Need https://git.busybox.net/busybox/commit/?id=d3539be8f27b8cbfdfee460fe08299158f08bcd9 on busybox 1.31.1

git cl -q && \
mkdir -p out.s390x && \
curl -LSso out.s390x/.config 'https://git.archlinux.org/svntogit/community.git/plain/trunk/config?h=packages/busybox' && \
make -j$(nproc) CROSS_COMPILE=s390x-linux-gnu- CONFIG_EXTRA_LDLIBS=resolv O=out.s390x oldconfig all

$ cat init
#!/bin/busybox sh

/bin/busybox mount -t proc proc /proc
/bin/busybox mount -t sysfs sysfs /sys

/bin/busybox --install -s

echo
echo
cat /proc/version
echo
echo
poweroff -f

mkdir -p bin sbin etc proc sys usr/bin usr/sbin
cp -v ... bin/busybox
find . -print0 | cpio --null -ov --format=newc > rootfs.cpio