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Re: [Qemu-devel] [PATCH 0/2] extboot reloaded.

From: Kevin O'Connor
Subject: Re: [Qemu-devel] [PATCH 0/2] extboot reloaded.
Date: Wed, 18 Nov 2009 20:27:54 -0500
User-agent: Mutt/1.5.19 (2009-01-05)

On Wed, Nov 18, 2009 at 08:42:10AM -0600, Anthony Liguori wrote:
> Kevin O'Connor wrote:
>> The order of hard drives is determined by the BCV list.  There is only
>> one entry in the IPL table for booting from a hard drive - selecting
>> "-boot c" prioritizes this IPL - hard drive booting always boots from
>> the "C" drive.  It's possible to tell SeaBIOS to map the C drive to
>> something other than the first item on the BCV list, but that
>> functionality is currently only available via the boot menu.
> This is the bit that confused me.

The IPL and BCV stuff is confusing.

An IPL is a way to boot the machine (floppy, hard drive, cdrom, BEVs
(eg, network card)).  A BCV is a hard drive.

So, there is only one IPL entry for booting from a hard drive, though
one may have many hard drives.

>Does an IPL entry for the hard disk  
> result in each BCV device being booted in order of BCV priority or is  
> there a means to have multiple IPL entries?  IOW, how would we support a  
> boot order of first hard disk, second hard disk, then CD-ROM?

With full compatibility, it's not possible to have a boot order of
first hard drive, then second hard drive.

The only compatible way to boot from a hard drive is to map that hard
drive to drive "C".  Once a drive is mapped, the mapping can't be
changed.  (Theoretically, seabios, could alter the mappings of
internally handled drives, but there is no way to remap a hard drive
added by an option rom.)

The BCV list is looked at in the last stage of POST - each drive is
mapped at that point.  The first drive on the list becomes the "C"
drive (unless one asks SeaBIOS to move a drive up by selecting it in
the boot menu - in which case that drive becomes drive "C").  Each
successive entry in the list becomes "D", "E", etc.

The IPL list is looked at during boot (after POST) and seabios
attempts each method in the list.  The "-boot" parameter instructs
seabios what order of IPL entries to try during boot.

As for adding two IPL entries for hard drives - it wouldn't be useful
because one can only boot from the "C" drive.

Hope this helps.

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