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

Kevin O'Connor kevin at koconnor.net
Tue Oct 12 02:08:08 CEST 2010

On Mon, Oct 11, 2010 at 12:18:55PM +0200, Gleb Natapov wrote:
> 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)

That makes sense to me.

We could update SeaBIOS to give a short unique name to every BEV and
BCV it finds based on the path to the device.  (For example, something
like "ata at 01:13.0 at 0", "usb at 1234:5678", "virtio at 01:13.0",
"rom at 01:13.0".)  Then qemu could pass in (via fw_cfg) a list of names
that the user wishes to boot from.  SeaBIOS can then prioritizes those
devices it finds that are also in the fw_cfg list.

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

As for qemu command line - maybe just use the current ",boot=on"
syntax, and have qemu map it into a "path name" for seabios?


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