Re: Completing big real mode emulation

Sheng Yang
2010-03-24T10:05:30Z

On Saturday 20 March 2010 23:00:49 Alexander Graf wrote:
> Am 20.03.2010 um 15:02 schrieb Mohammed Gamal <m.gamal005@gmail.com>:
> > On Sat, Mar 20, 2010 at 3:18 PM, Avi Kivity <avi@redhat.com> 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.
>
At least 10u2 installer has trouble. I had spent some time on it, finally
found it's due to ISOLINUX.

The basic issue is it assume that SS selector/base is unchanged when
enter/exit protect mode. At that time, I've cooked a hack workaround for it,
but didn't think it's proper to upstream.

--
regards
Yang, Sheng