77 lines
3.4 KiB
ReStructuredText
77 lines
3.4 KiB
ReStructuredText
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Managing device boot order with bootindex properties
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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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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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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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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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Example
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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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.. parsed-literal::
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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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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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Limitations
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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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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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Mixing bootindex and boot order parameters
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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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