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

[Alexander Graf]

On 20.03.2010, at 08:49, Avi Kivity wrote:

> On 03/19/2010 05:29 PM, Mohammed Gamal wrote:
>> Hello all,
>> As some of you might know, I've worked on supporting big real mode
>> emulation on VMX back in GSoC 2008. Looking at the Qemu GSoC ideas
>> list for this year, I found it among the possible ideas for a GSoC
>> project. I'd be interested in driving this feature towards completion,
>> and I have a few questions about it.
>> 
>> - The kernel-space modifications needed to detect an invalid guest
>> state on VMX and drive emulation from that point was almost complete.
>> The part that was missing the most, is that the kvm x86 emulator
>> wasn't complete and didn't support the entire instruction set. I've
>> seen that the emulator has been the focus of some recent patches
>> (namely by Gleb Natapov). Is there anything else required to get big
>> real mode to work correctly on KVM?
>>   
> 
> IIRC there are some bugs in invalid guest state detection, so it may need 
> some work.  We don't support interrupt injection during invalid guest state, 
> that will be tricky since it needs access to memory and interrupt injection 
> currently happens from atomic context.  Finally, there may still be missing 
> instructions.
> 
>> - Do we have other problems supporting big real mode on non-VMX
>> instruction sets? And do we have problems supporting it on the
>> userspace side?
>>   
> 
> No.

Interestingly enough Hannes just tried to boot a Windows 98 VM on SVM yesterday 
and failed, while the same VM worked (mostly) with -no-kvm. So apparently 
there's more missing to it than just big real mode.

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 
:-).


Alex