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

[Anthony Liguori]

Avi Kivity wrote:
> > In fact, when you chain-load with grub, you often have to "swap" 
> > device mappings which has the effect of hooking int13 and faking the 
> > primary drive for another OS.  See 
> > http://www.gnu.org/software/grub/manual/html_node/DOS_002fWindows.html
>
> But grub still has to issue a real int 13 for drive 0x81.  If we don't 
> expose it, it can't.  Until a real OS is loaded, the only way to 
> access a drive is through int 13 and if we don't expose it, it's 
> invisible.

extboot ignores any non-0x80 access today FWIW so this is already broken.

Only 0x80 is generally bootable.  This means even on modern systems, you 
still have per-adapter config menus that allow the user to configure 
which drive ends up getting 0x80.  It's awfully messy and not worth 
exposing to users.

> > All else aside, from a BIOS perspective, you can usually only boot 
> > from 0x80 and that can be mapped to different drives via BCV.
>
> There things you can do with a drive other than boot it.  For example, 
> access it via int 13.
>
> How will you access D:\ from DOS?

Let the user specify BCV priority, let the BIOS determine how to assign 
drive numbering.  It may turn out that bios drive number isn't stable 
but this is also true on real machines.

Regards,

Anthony Liguori