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+Managing device boot order with bootindex properties
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+====================================================
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+
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+QEMU can tell QEMU-aware guest firmware (like the x86 PC BIOS)
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+which order it should look for a bootable OS on which devices.
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+A simple way to set this order is to use the ``-boot order=`` option,
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+but you can also do this more flexibly, by setting a ``bootindex``
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+property on the individual block or net devices you specify
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+on the QEMU command line.
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+
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+The ``bootindex`` properties are used to determine the order in which
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+firmware will consider devices for booting the guest OS. If the
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+``bootindex`` property is not set for a device, it gets the lowest
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+boot priority. There is no particular order in which devices with no
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+``bootindex`` property set will be considered for booting, but they
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+will still be bootable.
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+
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+Some guest machine types (for instance the s390x machines) do
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+not support ``-boot order=``; on those machines you must always
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+use ``bootindex`` properties.
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+
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+There is no way to set a ``bootindex`` property if you are using
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+a short-form option like ``-hda`` or ``-cdrom``, so to use
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+``bootindex`` properties you will need to expand out those options
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+into long-form ``-drive`` and ``-device`` option pairs.
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+
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+Example
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+-------
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+
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+Let's assume we have a QEMU machine with two NICs (virtio, e1000) and two
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+disks (IDE, virtio):
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+
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+.. parsed-literal::
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+
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+ |qemu_system| -drive file=disk1.img,if=none,id=disk1 \\
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+ -device ide-hd,drive=disk1,bootindex=4 \\
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+ -drive file=disk2.img,if=none,id=disk2 \\
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+ -device virtio-blk-pci,drive=disk2,bootindex=3 \\
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+ -netdev type=user,id=net0 \\
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+ -device virtio-net-pci,netdev=net0,bootindex=2 \\
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+ -netdev type=user,id=net1 \\
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+ -device e1000,netdev=net1,bootindex=1
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+
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+Given the command above, firmware should try to boot from the e1000 NIC
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+first. If this fails, it should try the virtio NIC next; if this fails
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+too, it should try the virtio disk, and then the IDE disk.
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+
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+Limitations
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+-----------
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+
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+Some firmware has limitations on which devices can be considered for
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+booting. For instance, the PC BIOS boot specification allows only one
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+disk to be bootable. If boot from disk fails for some reason, the BIOS
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+won't retry booting from other disk. It can still try to boot from
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+floppy or net, though.
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+
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+Sometimes, firmware cannot map the device path QEMU wants firmware to
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+boot from to a boot method. It doesn't happen for devices the firmware
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+can natively boot from, but if firmware relies on an option ROM for
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+booting, and the same option ROM is used for booting from more then one
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+device, the firmware may not be able to ask the option ROM to boot from
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+a particular device reliably. For instance with the PC BIOS, if a SCSI HBA
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+has three bootable devices target1, target3, target5 connected to it,
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+the option ROM will have a boot method for each of them, but it is not
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+possible to map from boot method back to a specific target. This is a
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+shortcoming of the PC BIOS boot specification.
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+
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+Mixing bootindex and boot order parameters
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+------------------------------------------
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+
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+Note that it does not make sense to use the bootindex property together
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+with the ``-boot order=...`` (or ``-boot once=...``) parameter. The guest
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+firmware implementations normally either support the one or the other,
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+but not both parameters at the same time. Mixing them will result in
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+undefined behavior, and thus the guest firmware will likely not boot
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+from the expected devices.
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Peter Maydell