Commit 466b5864 authored by Gleb Natapov's avatar Gleb Natapov Committed by Anthony Liguori
Browse files

Add boot index documentation.

Signed-off-by: default avatarGleb Natapov <>
Signed-off-by: default avatarAnthony Liguori <>
parent cf8ce30d
= Bootindex propery =
Block and net devices have bootindex property. This property is used to
determine the order in which firmware will consider devices for booting
the guest OS. If the bootindex property is not set for a device, it gets
lowest boot priority. There is no particular order in which devices with
unset bootindex property will be considered for booting, but they will
still be bootable.
== Example ==
Lets assume we have QEMU machine with two NICs (virtio, e1000) and two
disks (IDE, virtio):
qemu -drive file=disk1.img,if=none,id=disk1
-device ide-drive,drive=disk1,bootindex=4
-drive file=disk2.img,if=none,id=disk2
-device virtio-blk-pci,drive=disk2,bootindex=3
-netdev type=user,id=net0 -device virtio-net-pci,netdev=net0,bootindex=2
-netdev type=user,id=net1 -device e1000,netdev=net1,bootindex=1
Given the command above, firmware should try to boot from the e1000 NIC
first. If this fails, it should try the virtio NIC next, if this fails
too, it should try the virtio disk, and then the IDE disk.
== Limitations ==
1. Some firmware has limitations on which devices can be considered for
booting. For instance, the PC BIOS boot specification allows only one
disk to be bootable. If boot from disk fails for some reason, the BIOS
won't retry booting from other disk. It still can try to boot from
floppy or net, though.
2. Sometimes, firmware cannot map the device path QEMU wants firmware to
boot from to a boot method. It doesn't happen for devices the firmware
can natively boot from, but if firmware relies on an option ROM for
booting, and the same option ROM is used for booting from more then one
device, the firmware may not be able to ask the option ROM to boot from
a particular device reliably. For instance with PC BIOS, if a SCSI HBA
has three bootable devices target1, target3, target5 connected to it,
the option ROM will have a boot method for each of them, but it is not
possible to map from boot method back to a specific target. This is a
shortcoming of PC BIOS boot specification.
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