[SeaBIOS] WinXP 32 installation failure with new SeaBIOS in QEMU

[Kevin O'Connor]

On Sun, Aug 19, 2012 at 02:39:04PM +0300, Avi Kivity wrote:
> On 08/19/2012 12:27 PM, Avi Kivity wrote:
> > On 08/17/2012 08:53 AM, Gerd Hoffmann wrote:
> >> This looks suspious.  vcpu migration issue?  Or just something missing
> >> in the trace?
> > Looks like tracing with a too-small buffer size.  I generally trace using
> > 
> >   trace-cmd record -e kvm -b 100000
> > 
> > and with fingers crossed.
> > 
> > The RET FAR instruction failure (which trace-cmd report decodes) is
> > probably not the original failure.
> 
> In fact it is, we mis-emulated stack operations when SS.B=0 but the
> address size was 32-bits, and there are bits set in the top 16 bits of ESP.
> 
> Why ESP has bits set in the top 16 bits is another question.  Is seabios
> polluting those bits?

During the WinXP install/boot, when it makes 16bit calls it only
resets the lower bits of %esp.  It expects the 16bit code to only use
%sp.  Quite spectacularly, WinXP requires the top bits of %esp to not
be mangled by the 16bit code.

SeaBIOS is very careful to not spuriously alter calling state.  This
includes backing up the top bits of %esp and restoring them on return.

-Kevin