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

Gleb Natapov gleb at redhat.com
Mon Oct 11 17:39:28 CEST 2010

On Mon, Oct 11, 2010 at 05:09:22PM +0200, Avi Kivity wrote:
>  On 10/11/2010 12:18 PM, 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)
> >
> >Will it cover all use cased? Any other ideas? Any ideas about qemu
> >command line syntax? May be somebody whats to implement it? :)
> Instead of fwcfg, we should store the boot order in the bios.  This
> allows seabios to implement persistent boot selection and control
> boot order from within the guest.
It is not "instead of" it is in a best case "in addition too". First of
all seabios does not have persistent storage currently and second I much
prefer specifying boot device from command line instead of navigating
bios menus. That what we have to do on real HW because there is not
other way to do it, but in virtualization we can do better.


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