Re: [RFC] Patch - Big real mode emulation

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, May 21, 2008 at 04:59:46PM +0300, Avi Kivity wrote:
> >Hello,
> >
> > Here is a patch that allows to boot OpenSuse-10.3. The problem with
> >Opensuse 10.3 is it uses a version of gfxboot that reads SS after
> >switching from real to protected mode, where SS contains an invalid
> >value, which VMX does not allow. 
> 
> Good to see progress on this issue.
> 
> >So this patch 
> >
> > 1) removes the code that writes sane value in SS in order to detect VM
> >entry failure due to CS.RPL != SS.RPL
> > 2) adds an handler to catch the VMentry failure
> >
> > The handler calls instruction's emulator and to boot opensuse we need
> >to emulate the following instructions:
> >
> >	ljmp   $0x18,$0x6e18
> >	mov    $0x20,%ax
> >	mov    %eax,%ds
> >	mov    %ss,%eax
> >	and    $0xffff,%esp
> >	shl    $0x4,%eax
> >	add    %eax,%esp
> >	mov    $0x8,%ax
> >	mov    %eax,%ss	
> >	-> At this point CS.RPL == SS.RPL
> >
> > There is an issue with the patch. When removing the SS patching we see
> >other problems. So to be able to still boot distribution that was
> >already able to boot we added a hack that allows to modify SS_SELECTOR
> >(as it was done previously) when emulation failed. The future solution
> >will be to emulate instruction that need to be emulated.
> >  
> 
> Which instructions are still problematic?

FreeDOS HIMEM uses ltr, ldt, loop, nop, does a task switch via jmp. But
that can come in later.