| = Bootindex property = |
| |
| 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. |
