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

[Steve Ofsthun <sofsthun@xxxxxxxxxxxxxxx>]

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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