[SeaBIOS] [Qemu-devel] [RFC] Passing boot order from qemu to seabios

Kevin Wolf kwolf at redhat.com
Mon Oct 11 13:01:51 CEST 2010

Am 11.10.2010 12:43, schrieb Gleb Natapov:
> On Mon, Oct 11, 2010 at 12:32:48PM +0200, Kevin Wolf wrote:
>> Am 11.10.2010 12:18, schrieb Gleb Natapov:
>>> 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?) 
>> Floppy? Yes, I think we do.
>>> 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)
>> If we had a qdev ID for all devices (which I think we should have
>> anyway), would this work or is a string not really handy enough?
> Given qdev ID how seabios knows what device it corresponds to?

Right, somehow I assumed that SeaBIOS already has some information about
disks, but now I see that this is exactly the problem you're talking
about. My suggestion wasn't really helpful then.

I think what you described is more or less the only way to do it then.


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