docs/system/s390x: Add a chapter about s390x boot devices

Booting on s390x is a little bit different compared to other architectures.
Let's add some information for people who are not yet used to this.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200806150507.12073-1-thuth@redhat.com>
[CH: minor wording tweaks]
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

docs/system/s390x/bootdevices.rst

@@ -0,0 +1,82 @@
+Boot devices on s390x
+=====================
+
+Booting with bootindex parameter
+--------------------------------
+
+For classical mainframe guests (i.e. LPAR or z/VM installations), you always
+have to explicitly specify the disk where you want to boot from (or "IPL" from,
+in s390x-speak -- IPL means "Initial Program Load"). In particular, there can
+also be only one boot device according to the architecture specification, thus
+specifying multiple boot devices is not possible (yet).
+
+So for booting an s390x guest in QEMU, you should always mark the
+device where you want to boot from with the ``bootindex`` property, for
+example::
+
+ qemu-system-s390x -drive if=none,id=dr1,file=guest.qcow2 \
+                   -device virtio-blk,drive=dr1,bootindex=1
+
+For booting from a CD-ROM ISO image (which needs to include El-Torito boot
+information in order to be bootable), it is recommended to specify a ``scsi-cd``
+device, for example like this::
+
+ qemu-system-s390x -blockdev file,node-name=c1,filename=... \
+                   -device virtio-scsi \
+                   -device scsi-cd,drive=c1,bootindex=1
+
+Note that you really have to use the ``bootindex`` property to select the
+boot device. The old-fashioned ``-boot order=...`` command of QEMU (and
+also ``-boot once=...``) is not supported on s390x.
+
+
+Booting without bootindex parameter
+-----------------------------------
+
+The QEMU guest firmware (the so-called s390-ccw bios) has also some rudimentary
+support for scanning through the available block devices. So in case you did
+not specify a boot device with the ``bootindex`` property, there is still a
+chance that it finds a bootable device on its own and starts a guest operating
+system from it. However, this scanning algorithm is still very rough and may
+be incomplete, so that it might fail to detect a bootable device in many cases.
+It is really recommended to always specify the boot device with the
+``bootindex`` property instead.
+
+This also means that you should avoid the classical short-cut commands like
+``-hda``, ``-cdrom`` or ``-drive if=virtio``, since it is not possible to
+specify the ``bootindex`` with these commands. Note that the convenience
+``-cdrom`` option even does not give you a real (virtio-scsi) CD-ROM device on
+s390x. Due to technical limitations in the QEMU code base, you will get a
+virtio-blk device with this parameter instead, which might not be the right
+device type for installing a Linux distribution via ISO image. It is
+recommended to specify a CD-ROM device via ``-device scsi-cd`` (as mentioned
+above) instead.
+
+
+Booting from a network device
+-----------------------------
+
+Beside the normal guest firmware (which is loaded from the file ``s390-ccw.img``
+in the data directory of QEMU, or via the ``-bios`` option), QEMU ships with
+a small TFTP network bootloader firmware for virtio-net-ccw devices, too. This
+firmware is loaded from a file called ``s390-netboot.img`` in the QEMU data
+directory. In case you want to load it from a different filename instead,
+you can specify it via the ``-global s390-ipl.netboot_fw=filename``
+command line option.
+
+The ``bootindex`` property is especially important for booting via the network.
+If you don't specify the the ``bootindex`` property here, the network bootloader
+firmware code won't get loaded into the guest memory so that the network boot
+will fail. For a successful network boot, try something like this::
+
+ qemu-system-s390x -netdev user,id=n1,tftp=...,bootfile=... \
+                   -device virtio-net-ccw,netdev=n1,bootindex=1
+
+The network bootloader firmware also has basic support for pxelinux.cfg-style
+configuration files. See the `PXELINUX Configuration page
+<https://wiki.syslinux.org/wiki/index.php?title=PXELINUX#Configuration>`__
+for details how to set up the configuration file on your TFTP server.
+The supported configuration file entries are ``DEFAULT``, ``LABEL``,
+``KERNEL``, ``INITRD`` and ``APPEND`` (see the `Syslinux Config file syntax
+<https://wiki.syslinux.org/wiki/index.php?title=Config>`__ for more
+information).

docs/system/target-s390x.rst

@@ -31,4 +31,5 @@ Architectural features
 ======================
 
 .. toctree::
+   s390x/bootdevices
    s390x/protvirt