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<section id="testing-bpf-on-s390">
<h1>Testing BPF on s390<a class="headerlink" href="#testing-bpf-on-s390" title="Link to this heading">¶</a></h1>
<section id="introduction">
<h2>1. Introduction<a class="headerlink" href="#introduction" title="Link to this heading">¶</a></h2>
<p>IBM Z are mainframe computers, which are descendants of IBM System/360 from
year 1964. They are supported by the Linux kernel under the name “s390”. This
document describes how to test BPF in an s390 QEMU guest.</p>
</section>
<section id="one-time-setup">
<h2>2. One-time setup<a class="headerlink" href="#one-time-setup" title="Link to this heading">¶</a></h2>
<p>The following is required to build and run the test suite:</p>
<blockquote>
<div><ul class="simple">
<li><p>s390 GCC</p></li>
<li><p>s390 development headers and libraries</p></li>
<li><p>Clang with BPF support</p></li>
<li><p>QEMU with s390 support</p></li>
<li><p>Disk image with s390 rootfs</p></li>
</ul>
</div></blockquote>
<p>Debian supports installing compiler and libraries for s390 out of the box.
Users of other distros may use debootstrap in order to set up a Debian chroot:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>sudo debootstrap \
--variant=minbase \
--include=sudo \
testing \
./s390-toolchain
sudo mount --rbind /dev ./s390-toolchain/dev
sudo mount --rbind /proc ./s390-toolchain/proc
sudo mount --rbind /sys ./s390-toolchain/sys
sudo chroot ./s390-toolchain
</pre></div>
</div>
<p>Once on Debian, the build prerequisites can be installed as follows:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>sudo dpkg --add-architecture s390x
sudo apt-get update
sudo apt-get install \
bc \
bison \
cmake \
debootstrap \
dwarves \
flex \
g++ \
gcc \
g++-s390x-linux-gnu \
gcc-s390x-linux-gnu \
gdb-multiarch \
git \
make \
python3 \
qemu-system-misc \
qemu-utils \
rsync \
libcap-dev:s390x \
libelf-dev:s390x \
libncurses-dev
</pre></div>
</div>
<p>Latest Clang targeting BPF can be installed as follows:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>git clone https://github.com/llvm/llvm-project.git
ln -s ../../clang llvm-project/llvm/tools/
mkdir llvm-project-build
cd llvm-project-build
cmake \
-DLLVM_TARGETS_TO_BUILD=BPF \
-DCMAKE_BUILD_TYPE=Release \
-DCMAKE_INSTALL_PREFIX=/opt/clang-bpf \
../llvm-project/llvm
make
sudo make install
export PATH=/opt/clang-bpf/bin:$PATH
</pre></div>
</div>
<p>The disk image can be prepared using a loopback mount and debootstrap:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>qemu-img create -f raw ./s390.img 1G
sudo losetup -f ./s390.img
sudo mkfs.ext4 /dev/loopX
mkdir ./s390.rootfs
sudo mount /dev/loopX ./s390.rootfs
sudo debootstrap \
--foreign \
--arch=s390x \
--variant=minbase \
--include=" \
iproute2, \
iputils-ping, \
isc-dhcp-client, \
kmod, \
libcap2, \
libelf1, \
netcat, \
procps" \
testing \
./s390.rootfs
sudo umount ./s390.rootfs
sudo losetup -d /dev/loopX
</pre></div>
</div>
</section>
<section id="compilation">
<h2>3. Compilation<a class="headerlink" href="#compilation" title="Link to this heading">¶</a></h2>
<p>In addition to the usual Kconfig options required to run the BPF test suite, it
is also helpful to select:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>CONFIG_NET_9P=y
CONFIG_9P_FS=y
CONFIG_NET_9P_VIRTIO=y
CONFIG_VIRTIO_PCI=y
</pre></div>
</div>
<p>as that would enable a very easy way to share files with the s390 virtual
machine.</p>
<p>Compiling kernel, modules and testsuite, as well as preparing gdb scripts to
simplify debugging, can be done using the following commands:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>make ARCH=s390 CROSS_COMPILE=s390x-linux-gnu- menuconfig
make ARCH=s390 CROSS_COMPILE=s390x-linux-gnu- bzImage modules scripts_gdb
make ARCH=s390 CROSS_COMPILE=s390x-linux-gnu- \
-C tools/testing/selftests \
TARGETS=bpf \
INSTALL_PATH=$PWD/tools/testing/selftests/kselftest_install \
install
</pre></div>
</div>
</section>
<section id="running-the-test-suite">
<h2>4. Running the test suite<a class="headerlink" href="#running-the-test-suite" title="Link to this heading">¶</a></h2>
<p>The virtual machine can be started as follows:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>qemu-system-s390x \
-cpu max,zpci=on \
-smp 2 \
-m 4G \
-kernel linux/arch/s390/boot/compressed/vmlinux \
-drive file=./s390.img,if=virtio,format=raw \
-nographic \
-append 'root=/dev/vda rw console=ttyS1' \
-virtfs local,path=./linux,security_model=none,mount_tag=linux \
-object rng-random,filename=/dev/urandom,id=rng0 \
-device virtio-rng-ccw,rng=rng0 \
-netdev user,id=net0 \
-device virtio-net-ccw,netdev=net0
</pre></div>
</div>
<p>When using this on a real IBM Z, <code class="docutils literal notranslate"><span class="pre">-enable-kvm</span></code> may be added for better
performance. When starting the virtual machine for the first time, disk image
setup must be finalized using the following command:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>/debootstrap/debootstrap --second-stage
</pre></div>
</div>
<p>Directory with the code built on the host as well as <code class="docutils literal notranslate"><span class="pre">/proc</span></code> and <code class="docutils literal notranslate"><span class="pre">/sys</span></code>
need to be mounted as follows:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>mkdir -p /linux
mount -t 9p linux /linux
mount -t proc proc /proc
mount -t sysfs sys /sys
</pre></div>
</div>
<p>After that, the test suite can be run using the following commands:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>cd /linux/tools/testing/selftests/kselftest_install
./run_kselftest.sh
</pre></div>
</div>
<p>As usual, tests can be also run individually:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>cd /linux/tools/testing/selftests/bpf
./test_verifier
</pre></div>
</div>
</section>
<section id="debugging">
<h2>5. Debugging<a class="headerlink" href="#debugging" title="Link to this heading">¶</a></h2>
<p>It is possible to debug the s390 kernel using QEMU GDB stub, which is activated
by passing <code class="docutils literal notranslate"><span class="pre">-s</span></code> to QEMU.</p>
<p>It is preferable to turn KASLR off, so that gdb would know where to find the
kernel image in memory, by building the kernel with:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>RANDOMIZE_BASE=n
</pre></div>
</div>
<p>GDB can then be attached using the following command:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>gdb-multiarch -ex 'target remote localhost:1234' ./vmlinux
</pre></div>
</div>
</section>
<section id="network">
<h2>6. Network<a class="headerlink" href="#network" title="Link to this heading">¶</a></h2>
<p>In case one needs to use the network in the virtual machine in order to e.g.
install additional packages, it can be configured using:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>dhclient eth0
</pre></div>
</div>
</section>
<section id="links">
<h2>7. Links<a class="headerlink" href="#links" title="Link to this heading">¶</a></h2>
<p>This document is a compilation of techniques, whose more comprehensive
descriptions can be found by following these links:</p>
<ul class="simple">
<li><p><a class="reference external" href="https://wiki.debian.org/EmDebian/CrossDebootstrap">Debootstrap</a></p></li>
<li><p><a class="reference external" href="https://wiki.debian.org/Multiarch/HOWTO">Multiarch</a></p></li>
<li><p><a class="reference external" href="https://llvm.org/docs/CMake.html">Building LLVM</a></p></li>
<li><p><a class="reference external" href="https://wiki.gentoo.org/wiki/Embedded_Handbook/General/Cross-compiling_the_kernel">Cross-compiling the kernel</a></p></li>
<li><p><a class="reference external" href="https://wiki.qemu.org/Documentation/Platforms/S390X">QEMU s390x Guest Support</a></p></li>
<li><p><a class="reference external" href="https://wiki.qemu.org/Documentation/9psetup">Plan 9 folder sharing over Virtio</a></p></li>
<li><p><a class="reference external" href="https://wiki.osdev.org/Kernel_Debugging#Use_GDB_with_QEMU">Using GDB with QEMU</a></p></li>
</ul>
</section>
</section>
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