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Re: [Xen-devel] HVMAssist BIOS boot device problem

To: Stephen Quintero <stephen@xxxxxxxxx>
Subject: Re: [Xen-devel] HVMAssist BIOS boot device problem
From: Steve Ofsthun <sofsthun@xxxxxxxxxxxxxxx>
Date: Tue, 08 Jul 2008 10:05:10 -0400
Cc: Apoorva Kulkarni <Apoorva@xxxxxxxxxx>, Minh Quach <minh@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, Eric Tessler <eric@xxxxxxxxx>
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Stephen Quintero wrote:
Keir,

Thank you for your reply.

I agree it should be deterministic, according to the enumeration of the
devices (hda-hdd), but, alas, it is not.

Realize that Xen (QEMU BIOS) is only determining which MBR is loaded and executed.  After that you 
are at the mercy of the boot loader in question.  Windows in particular doesn't "support" 
multiple visible system partitions, or rather, doesn't specify the selection of the appropriate 
system partition.  In your example B below, if you destroy the boot block on hda, you will fail to 
boot anything.  If you "hide" all windows partitions on hdb/hdc/hdd, you will indeed boot 
the system volume on hda.  This is exactly what many multiboot boot loaders do.  The partition 
tables are usually modified in memory only, not on the physical disk itself.

Steve

See example B in my original post where hda|hdb|hdc all contain active
partitions but the guest boots from hdc.

See also example A in my post where hda is always a bootable image but
sometimes the guest boots from hdb (you may not easily recreate this
scenario yourself, but example B is simple to reproduce).

Stephen


-----Original Message-----
From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx] Sent: Tuesday, July 08, 2008 2:12 AM
To: Stephen Quintero; xen-devel@xxxxxxxxxxxxxxxxxxx
Cc: Apoorva Kulkarni; Minh Quach; Eric Tessler
Subject: Re: [Xen-devel] HVMAssist BIOS boot device problem

On 8/7/08 01:24, "Stephen Quintero" <stephen@xxxxxxxxx> wrote:

Is there any method to cause an HVM guest to reliably boot from hda?

Any insight would be appreciated.

Reading the BIOS code, my understanding is that 'boot from hard disk'
means
to boot from the first connected hard drive enumerated in the following
order: primary master, primary slave, secondary master, secondary slave
(i.e., hda-hdd). I confirmed this with a few simple tests -- attach a
bootable image to hdd boots fine, but if I connect a non-bootable image
on
any of hha-hdc then the system fails to boot. This is all entirely
deterministic. Selection of boot device should not be random, nor change
across reboots.

 -- Keir



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