[Qemu-devel] Re: Completing big real mode emulation

[Avi Kivity]

On 03/20/2010 10:55 AM, Alexander Graf wrote:
> > > I'd say that a GSoC project would rather focus on making a guest OS work than 
> > > working on generic big real mode. Having Windows 98 support is way more visible 
> > > to the users. And hopefully more fun to implement too, as it's a visible goal 
> > > :-).
> > >
> > >        
> > Big real mode allows you to boot various OSes, such as that old Ubuntu/SuSE 
> > boot loader which triggered the whole thing.
> >      
> I thought legacy Windows uses it too?
>    

IIRC even current Windows (last I checked was XP, but it's probably true 
for newer) invokes big real mode inadvertently.  All it takes is not to 
clear fs and gs while switching to real mode.  It works because the real 
mode code never uses gs and fs (i.e. while we are technically in big 
real mode, the guest never relies on this), and because there are enough 
hacks in vmx.c to make it work (restoring fs and gs after the switch 
back).  IIRC there are other cases of invalid guest state that we hack 
into place during mode switches.

> Either way - then we should make the goal of the project to support those old 
> boot loaders. IMHO it should contain visibility. Doing theoretical stuff is 
> just less fun for all parties. Or does that stuff work already?
>    

Mostly those old guests aged beyond usefulness.  They are still broken, 
but nobody installs new images.  Old images installed via workarounds work.

Goals for this task could include:

 - get those older guests working
 - get emulate_invalid_guest_state=1 to work on all supported guests
 - switch to emulate_invalid_guest_state=1 as the default
 - drop the code supporting emulate_invalid_guest_state=0 eventually

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