52 lines
2.4 KiB
Text
52 lines
2.4 KiB
Text
= Bootindex property =
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Block and net devices have bootindex property. This property is used to
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determine the order in which firmware will consider devices for booting
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the guest OS. If the bootindex property is not set for a device, it gets
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lowest boot priority. There is no particular order in which devices with
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unset bootindex property will be considered for booting, but they will
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still be bootable.
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== Example ==
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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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qemu -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 -device virtio-net-pci,netdev=net0,bootindex=2
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-netdev type=user,id=net1 -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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1. 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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2. 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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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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