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

Anthony Liguori anthony at codemonkey.ws
Mon Oct 11 19:01:58 CEST 2010

On 10/11/2010 10:52 AM, Stefan Hajnoczi wrote:
> 2010/10/11 Gleb Natapov<gleb at redhat.com>:
>> On Mon, Oct 11, 2010 at 01:48:09PM +0100, Stefan Hajnoczi wrote:
>>> On Mon, Oct 11, 2010 at 12:16 PM, Bernhard Kohl<bernhard.kohl at nsn.com>  wrote:
>>>> Am 11.10.2010 12:18, schrieb ext 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?) 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?
>>>> I think this also applies to network booting via gPXE. Usually our VMs
>>>> have 4 NICs, mixed virtio-net and PCI pass-through. 2 of the NICs shall
>>>> be used for booting, even if there are hard disks or floppy disks
>>>> connected. This scenario is currently almost impossible to configure.
>>> Here is a gPXE to support fw_cfg.  You can pass gPXE script files from
>>> the host to gPXE inside the guest.  This means you can boot specific
>>> NICs:
>>> http://patchwork.ozlabs.org/patch/43777/
>>> Just wanted to post the link because it is related to the gPXE side of
>>> this discussion.
>> Don't we load gPXE for each NIC and seabios passes PCI device to boot from
>> when it invokes one of them?
> SeaBIOS may do that but gPXE internally just probes all PCI devices.
> It does not take advantage of the PCI bus/addr/fn that was passed to
> the option ROM.  A gPXE instance will try booting from each available
> NIC in sequence.

It still registers a BEV entry though, no?

Does it at least try to boot from the PCI bus/addr/fn of the selected 
BEV entry?


Anthony Liguori

> Stefan

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