[SeaBIOS] [RFC] Passing boot order from qemu to seabios

Gleb Natapov gleb at redhat.com
Mon Oct 11 12:18:55 CEST 2010

Currently if VM is started with multiple disks it is almost impossible to
guess which one of them will be used as boot device especially if there
is a mix of ATA/virtio/SCSI devices. Essentially BIOS decides the order
and without looking into the code you can't tell what the order will
be (and in qemu-kvm if boot=on is used it brings even more havoc). We
should allow fine-grained control of boot order from qemu command line,
or as a minimum control what device will be used for booting.

To do that along with inventing syntax to specify boot order on qemu
command line we need to communicate boot order to seabios via fw_cfg
interface. For that we need to have a way to unambiguously specify a
disk from qemu to seabios.  PCI bus address is not enough since not all
devices are PCI (do we care about them?) and since one PCI device may
control more then one disk (ATA slave/master, SCSI LUNs). We can do what
EDD specification does. Describe disk as:
    bus type (isa/pci),
    address on a bus (16 bit base address for isa, b/s/f for pci)
    device type (ATA/SCSI/VIRTIO)
    device path (slave/master for ATA, LUN for SCSI, nothing for virtio)

Will it cover all use cased? Any other ideas? Any ideas about qemu
command line syntax? May be somebody whats to implement it? :)


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