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

[Avi Kivity <avi@redhat.com>]

On 09/08/2009 10:46 PM, Anthony Liguori wrote:
> Avi Kivity wrote:
>> The bios exports the "bios drives".  You can tell grub to load a 
>> kernel from the first partition of the second disk: (hd1,0) or even 
>> chain-load another boot loaded from another disk.
>
> The whole point of device.map is that grub doesn't intrinsically know 
> how to map from bios drives to real drives. 

device.map is a wonderful thing, but unrelated to the bios exposing 
drive 0x81.

> 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.

> 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?

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.