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@01:13.0@0", "usb@1234:5678", "virtio@01:13.0", "rom@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?
-Kevin