Re: [Qemu-devel] Re: [PATCH 0/2] port over extboot from kvm

[Kevin O'Connor <kevin@koconnor.net>]

On Tue, Sep 08, 2009 at 07:05:15PM +0300, Avi Kivity wrote:
> On 09/08/2009 05:32 PM, Anthony Liguori wrote:
>> x86 boot is strange.  The BIOS boots from the "first hard disk"  What  
>> extboot allows you do to is redefine to the bios what constitutes the  
>> "first hard disk".
>
> It's not just booting - it's the int 0x13 interface.  Grub for example  
> continues to use it and can boot from the second hard disk if desired.

Some time back, I tried implementing booting from the second hard
drive by passing 0x81 in "dl" during the boot.  All the boot loaders I
tested failed to work.  I think grub may have worked if I
re-configured it, but I figured it wasn't worth it due to the
confusion it would cause.

So, the experience suggested to me that bootloaders today still
require the boot harddrive to be mapped to 0x80.  (Though,
interestingly, cdrom booting is okay with any drive id..)

>> For instance, it would not be possible to define a boot sequence of  
>> "first virtio disk, then second virtio disk, then cdrom" because we  
>> can't present two disks as the first hard disk.
>>
>> If extboot supported BCV and our bios did, the above would be possible.

FWIW, SeaBIOS does support BCVs.  SeaBIOS will find all the drives and
present them on the boot menu - the user can then select which drive
to boot from (either floppy, any number of cdroms, any of the builtin
or external "BCV" hard drives).  For floppy and hard drive booting the
selection is implemented by manipulating the drive mappings.

Also, as an aside, it seams likely that USB booting will be added to
SeaBIOS (either directly or as an option rom).  I'm not sure if this
has any overlap with extboot.

-Kevin