Re: Completing big real mode emulation

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Am 20.03.2010 um 15:02 schrieb Mohammed Gamal <m.gamal005@xxxxxxxxx>:

On Sat, Mar 20, 2010 at 3:18 PM, Avi Kivity <avi@xxxxxxxxxx> wrote:
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

To this end I guess the next logical step is to compile a list of
guests that are currently not working/work with hacks only, and get
them working. Here are some suggestions:
- MINIX 3.1.6 (developers have been recently filing bug reports
because of boot failures)
- Win XP with emulation enabled
- FreeDOS with memory extenders

Any other guests you'd like to see on this list?

I remember old openSUSE iso bootloaders had issues. I think it was around 10.3, but might have been earlier.

Alex
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