This is assuming a modern Ubunt distro. What we will do here is to install the toolchain that is needed to cross-compile for the target architecture.
[!WARNING] don’t go crazy in installed foreign arch packages, it may mess some of your system symlinks. Probably better off doing this in a container….
XPLATFORM="s390x"
XARCH="s390"
# Set up repo for s390x, it is only available from the `ports` repo.
cat <<EOF >> /etc/apt/sources.list.d/s390x.list
deb [arch=s390x] http://ports.ubuntu.com/ubuntu-ports mantic main restricted
deb [arch=s390x] http://ports.ubuntu.com/ubuntu-ports mantic-updates main restricted
EOF
# Add the architecture
sudo dpkg --add-architecture s390x
apt install g{cc,++}-"${XARCH}-linux-gnu" {libelf-dev,libssl-dev,pkgconf}:s390x
We need to apply xcompile_bpf_selftest.diff to the kernel tree in order to be able to
run bpftool for the target architecture during the skeleton generation.
In turn, this requires support for the foreign architecture with binfmt. For new Ubuntu systems, it is just a matter
of installing
sudo apt-get install -y qemu-user-static
Then you can see the different architecture supported by looking at:
$ ls /proc/sys/fs/binfmt_misc/
llvm-16-runtime.binfmt qemu-alpha qemu-cris qemu-loongarch64 qemu-mips qemu-mipsel qemu-ppc qemu-riscv32 qemu-sh4 qemu-sparc32plus qemu-xtensaeb
python3.11 qemu-arm qemu-hexagon qemu-m68k qemu-mips64 qemu-mipsn32 qemu-ppc64 qemu-riscv64 qemu-sh4eb qemu-sparc64 register
qemu-aarch64 qemu-armeb qemu-hppa qemu-microblaze qemu-mips64el qemu-mipsn32el qemu-ppc64le qemu-s390x qemu-sparc qemu-xtensa status
KBUILD_OUTPUT_DIR="/tmp/kbuild-${XPLATFORM}"
mkdir "${KBUILD_OUTPUT_DIR}"
cat tools/testing/selftests/bpf/config{,.vm,.${XPLATFORM}} > ${KBUILD_OUTPUT_DIR}/.config
make ARCH="${XARCH}" CROSS_COMPILE="${XPLATFORM}-linux-gnu-" O="${KBUILD_OUTPUT_DIR}" -j$((4 * $(nproc))) olddefconfig
make ARCH="${XARCH}" CROSS_COMPILE="${XPLATFORM}-linux-gnu-" O="${KBUILD_OUTPUT_DIR}" -j$((4 * $(nproc))) all
make ARCH="${XARCH}" CROSS_COMPILE="${XPLATFORM}-linux-gnu-" O="${KBUILD_OUTPUT_DIR}" -j$((4 * $(nproc))) -C tools/testing/selftests/bpf
Building selftest with clang:
make ARCH="${XARCH}" CROSS_COMPILE="${XPLATFORM}-linux-gnu-" O="${KBUILD_OUTPUT_DIR}" -j$((4 * $(nproc))) CLANG=clang-16 LLC=llc-16 LLVM_STRIP=llvm-strip-16 VMLINUX_BTF="${KBUILD_OUTPUT_DIR}/vmlinux" VMLINUX_H= -C tools/testing/selftests/bpf
In order to run the kernel in a VM we need a rootfs for s390x of the same Ubuntu version to avoid library mismatch.
docker2rootfs -R registry-1.docker.io -i s390x/ubuntu -r 23.10 -o /tmp/s390x_rootfs
# chroot and install a few useful packages
cp /etc/resolv.conf /tmp/s390x_rootfs/etc/
sudo chroot /tmp/s390x_rootfs/
# Within chroot
mount -t devtmpfs -o nosuid,noexec dev /dev
mount -t tmpfs tmpfs /tmp
apt update
DEBIAN_FRONTEND=noninteractive apt-get install -y qemu-guest-agent ethtool keyutils iptables gawk libelf1 zlib1g libssl3 iproute2 finit-sysv
umount /tmp /dev
# exit chroot
exit
vmtest -k "${KBUILD_OUTPUT_DIR}/arch/s390/boot/bzImage" -r /tmp/s390x_rootfs/ -a s390x "uname -m" | cat