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128 lines
6.2 KiB
ReStructuredText
128 lines
6.2 KiB
ReStructuredText
Boot devices on s390x
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=====================
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Booting with bootindex parameter
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--------------------------------
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For classical mainframe guests (i.e. LPAR or z/VM installations), you always
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have to explicitly specify the disk where you want to boot from (or "IPL" from,
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in s390x-speak -- IPL means "Initial Program Load").
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So for booting an s390x guest in QEMU, you should always mark the
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device where you want to boot from with the ``bootindex`` property, for
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example::
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qemu-system-s390x -drive if=none,id=dr1,file=guest.qcow2 \
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-device virtio-blk,drive=dr1,bootindex=1
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Multiple devices may have a bootindex. The lowest bootindex is assigned to the
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device to IPL first. If the IPL fails for the first, the device with the second
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lowest bootindex will be tried and so on until IPL is successful or there are no
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remaining boot devices to try.
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For booting from a CD-ROM ISO image (which needs to include El-Torito boot
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information in order to be bootable), it is recommended to specify a ``scsi-cd``
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device, for example like this::
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qemu-system-s390x -blockdev file,node-name=c1,filename=... \
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-device virtio-scsi \
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-device scsi-cd,drive=c1,bootindex=1
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Note that you really have to use the ``bootindex`` property to select the
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boot device. The old-fashioned ``-boot order=...`` command of QEMU (and
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also ``-boot once=...``) is not supported on s390x.
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Booting without bootindex parameter
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-----------------------------------
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The QEMU guest firmware (the so-called s390-ccw bios) has also some rudimentary
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support for scanning through the available block devices. So in case you did
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not specify a boot device with the ``bootindex`` property, there is still a
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chance that it finds a bootable device on its own and starts a guest operating
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system from it. However, this scanning algorithm is still very rough and may
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be incomplete, so that it might fail to detect a bootable device in many cases.
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It is really recommended to always specify the boot device with the
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``bootindex`` property instead.
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This also means that you should avoid the classical short-cut commands like
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``-hda``, ``-cdrom`` or ``-drive if=virtio``, since it is not possible to
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specify the ``bootindex`` with these commands. Note that the convenience
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``-cdrom`` option even does not give you a real (virtio-scsi) CD-ROM device on
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s390x. Due to technical limitations in the QEMU code base, you will get a
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virtio-blk device with this parameter instead, which might not be the right
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device type for installing a Linux distribution via ISO image. It is
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recommended to specify a CD-ROM device via ``-device scsi-cd`` (as mentioned
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above) instead.
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Selecting kernels with the ``loadparm`` property
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------------------------------------------------
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The ``s390-ccw-virtio`` machine supports the so-called ``loadparm`` parameter
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which can be used to select the kernel on the disk of the guest that the
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s390-ccw bios should boot. When starting QEMU, it can be specified like this::
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qemu-system-s390x -machine s390-ccw-virtio,loadparm=<string>
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The first way to use this parameter is to use the word ``PROMPT`` as the
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``<string>`` here. In that case the s390-ccw bios will show a list of
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installed kernels on the disk of the guest and ask the user to enter a number
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to chose which kernel should be booted -- similar to what can be achieved by
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specifying the ``-boot menu=on`` option when starting QEMU. Note that the menu
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list will only show the names of the installed kernels when using a DASD-like
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disk image with 4k byte sectors. On normal SCSI-style disks with 512-byte
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sectors, there is not enough space for the zipl loader on the disk to store
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the kernel names, so you only get a list without names here.
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The second way to use this parameter is to use a number in the range from 0
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to 31. The numbers that can be used here correspond to the numbers that are
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shown when using the ``PROMPT`` option, and the s390-ccw bios will then try
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to automatically boot the kernel that is associated with the given number.
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Note that ``0`` can be used to boot the default entry. If the machine
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``loadparm`` is not assigned a value, then the default entry is used.
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By default, the machine ``loadparm`` applies to all boot devices. If multiple
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devices are assigned a ``bootindex`` and the ``loadparm`` is to be different
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between them, an independent ``loadparm`` may be assigned on a per-device basis.
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An example guest using per-device ``loadparm``::
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qemu-system-s390x -drive if=none,id=dr1,file=primary.qcow2 \
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-device virtio-blk,drive=dr1,bootindex=1 \
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-drive if=none,id=dr2,file=secondary.qcow2 \
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-device virtio-blk,drive=dr2,bootindex=2,loadparm=3
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In this case, the primary boot device will attempt to IPL using the default
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entry (because no ``loadparm`` is specified for this device or for the
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machine). If that device fails to boot, the secondary device will attempt to
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IPL using entry number 3.
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If a ``loadparm`` is specified on both the machine and a device, the per-device
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value will superseded the machine value. Per-device ``loadparm`` values are
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only used for devices with an assigned ``bootindex``. The machine ``loadparm``
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is used when attempting to boot without a ``bootindex``.
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Booting from a network device
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-----------------------------
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The firmware that ships with QEMU includes a small TFTP network bootloader
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for virtio-net-ccw devices. The ``bootindex`` property is especially
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important for booting via the network. If you don't specify the ``bootindex``
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property here, the network bootloader won't be taken into consideration and
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the network boot will fail. For a successful network boot, try something
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like this::
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qemu-system-s390x -netdev user,id=n1,tftp=...,bootfile=... \
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-device virtio-net-ccw,netdev=n1,bootindex=1
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The network bootloader also has basic support for pxelinux.cfg-style
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configuration files. See the `PXELINUX Configuration page
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<https://wiki.syslinux.org/wiki/index.php?title=PXELINUX#Configuration>`__
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for details how to set up the configuration file on your TFTP server.
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The supported configuration file entries are ``DEFAULT``, ``LABEL``,
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``KERNEL``, ``INITRD`` and ``APPEND`` (see the `Syslinux Config file syntax
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<https://wiki.syslinux.org/wiki/index.php?title=Config>`__ for more
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information).
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