Cross-compiling with debian stretch

Debian stretch comes with cross-compiler packages for selected architectures:

 $ apt-cache search cross-build-essential
crossbuild-essential-arm64 - Informational list of cross-build-essential packages for
crossbuild-essential-armel - ...
crossbuild-essential-armhf - ...
crossbuild-essential-mipsel - ...
crossbuild-essential-powerpc - ...
crossbuild-essential-ppc64el - ...
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Lets have a quick exact steps guide. But first - while you can use do all this in your desktop PC rootfs, it is more wise to contain yourself. Fortunately, Debian comes with a container tool out of box:

sudo debootstrap stretch /var/lib/container/stretch http://deb.debian.org/debian
echo "strech_cross" | sudo tee /var/lib/container/stretch/etc/debian_chroot
sudo systemd-nspawn -D /var/lib/container/stretch

Then we set up cross-building enviroment for arm64 inside the container:

# Tell dpkg we can install arm64
dpkg --add-architecture arm64
# Add src line to make "apt-get source" work
echo "deb-src http://deb.debian.org/debian stretch main" >> /etc/apt/sources.list
apt-get update
# Install cross-compiler and other essential build tools
apt install --no-install-recommends build-essential crossbuild-essential-arm64

Now we have a nice build enviroment, lets choose something more complicated than the usual kernel/BusyBox to cross-build, qemu:

# Get qemu sources from debian
apt-get source qemu
cd qemu-*
# New in stretch: build-dep works in unpacked source tree 
apt-get build-dep -a arm64 .
# Cross-build Qemu for arm64
dpkg-buildpackage -aarm64 -j6 -b

Now that works perfectly for Qemu. For other packages, challenges may appear. For example you may have to se "nocheck" flag to skip build-time unit tests. Or some of the build-dependencies may not be multiarch-enabled. So work continues :)

Using networkd for kvm tap networking

Setting up basic systemd-network was recently described by Joachim, and the post inspired me to try it as well. The twist is that in my case I need a bridge for my KVM with Lava server and arm/aarch64 qemu system emulators...

For background, qemu/kvm support a few ways to provide network to guests. The default is user networking, which requires no privileges, but is slow and based on ancient SLIRP code. The other common option is tap networking, which is fast, but complicated to set up. Turns out, with networkd and qemu bridge helper, tap is easy to set up.

$ for file in /etc/systemd/network/*; do echo $file; cat $file; done
/etc/systemd/network/eth.network
[Match]
Name=eth1
[Network]
Bridge=br0

/etc/systemd/network/kvm.netdev
[NetDev]
Name=br0
Kind=bridge

/etc/systemd/network/kvm.network
[Match]
Name=br0
[Network]
DHCP=yes

Diverging from Joachims simple example, we replaced "DHCP=yes" with "Bridge=br0". Then we proceed to define the bridge (in the kvm.netdev) and give it an ip via dhcp in kvm.network. From the kvm side, if you haven't used the bridge helper before, you need to give the helper permissions (setuid root or cap_net_admin) to create a tap device to attach on the bridge. The helper needs an configuration file to tell what bridge it may meddle with.

# cat > /etc/qemu/bridge.conf <<__END__
allow br0
__END__
# setcap cap_net_admin=ep /usr/lib/qemu/qemu-bridge-helper

Now we can start kvm with bridge networking as easily as with user networking:

$ kvm -m 2048 -drive file=jessie.img,if=virtio -net bridge -net nic,model=virtio -serial stdio

The manpages systemd.network(5) and systemd.netdev(5) do a great job explaining the files. Qemu/kvm networking docs are unfortunately not as detailed.