Cross compilation:
------------------

The standard way of cross compiling with cmake is to give cmake a "toolchain
file" that describes the compiler prefix and where it can find libraries for
the target system, we provide an example in cmake/cross-aarch64.cmake.

This obviously requires installing a toolchain and a rootfs with target
libraries to link against.

In addition, mckernel borrows the Kbuild system from linux kernel, which also
makes the assumption that you can run generated executables (Kbuild uses various
scripts around module building and does not make the distinction between build
target and host target); you can get this working by setting up qemu-user on your
machine which the kernel will transparently use through binfmt magic when trying
to execute other arch binaries.

# yum install gcc-aarch64-linux-gnu qemu-user
(dnf brings in --forcearch, we use this to setup the sysroot ; there are other
ways of building one. It is available on el7 in extras.)
# yum install dnf dnf-plugins-core
(install these separately because most scripts cannot be run, and dependency hell means we need them first)
# dnf download --releasever=7 --forcearch=aarch64 filesystem centos-release
# rpm --root /usr/aarch64-linux-gnu/sys-root --ignorearch --nodeps -ivh filesystem-*.rpm
# rpm --root /usr/aarch64-linux-gnu/sys-root --ignorearch --nodeps -ivh centos-release-*.rpm
# rpm --root /usr/aarch64-linux-gnu/sys-root --import /usr/aarch64-linux-gnu/sys-root/etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
# rpm --root /usr/aarch64-linux-gnu/sys-root --import /usr/aarch64-linux-gnu/sys-root/etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7-aarch64
# dnf install --releasever=7  --forcearch=aarch64 --installroot=/usr/aarch64-linux-gnu/sys-root/ --setopt=tsflags=noscripts glibc-devel kernel-devel numactl-devel systemd-devel binutils-devel

(el7 lacks a binfmt for aarch64... fix this)
# echo ':qemu-aarch64:M::\x7fELF\x02\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\xb7\x00:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff:/usr/bin/qemu-aarch64:' > /etc/binfmt.d/qemu-aarch64-dynamic.conf
# systemctl restart systemd-binfmt

(optional) test your setup
# gcc -xc - <<<'#include <stdio.h>'$'\n''int main() { printf("ok\n"); return 0; }'
# export QEMU_LD_PREFIX=/usr/aarch64-linux-gnu/sys-root
# file a.out
a.out: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 3.7.0, BuildID[sha1]=5ff445d3353cad2dae0a22550fe4cc572287dd90, not stripped
# ./a.out
ok

finally, build mckernel!
# mkdir build; cd build
# export QEMU_LD_PREFIX=/usr/aarch64-linux-gnu/sys-root
# cmake -DCMAKE_INSTALL_PREFIX=/tmp/install-aarch64 -DCMAKE_TOOLCHAIN_FILE=../cmake/cross-aarch64.cmake -DUNAME_R=4.14.0-115.2.2.el7a.aarch64 -DKERNEL_DIR=/usr/aarch64-linux-gnu/sys-root/usr/src/kernels/4.14.0-115.2.2.el7a.aarch64/ -DBUILD_TARGET=smp-arm64 ..
# make -j