[Qemu-devel] Re: Seabios dislikes -M isapc

[Sebastian Herbszt]

Kevin O'Connor wrote:
> On Mon, Feb 08, 2010 at 08:09:17PM +0100, Jan Kiszka wrote:
> > Sebastian Herbszt wrote:
> > > Jan Kiszka wrote:
> > >> Hi,
> > >>
> > >> Seabios seems to have some assumptions built in that break when -M isapc
> > >> is selected. Is this supposed to work or is isapc about to die?
> > > 
> > > SeaBIOS doesn't POST if the F-segment is not writeable [1]. A possible, but IMO
> > > wrong fix was posted on the list [2].
> > > 
> > > [1] http://lists.gnu.org/archive/html/qemu-devel/2009-11/msg01188.html
> > > [2] http://lists.gnu.org/archive/html/qemu-devel/2009-12/msg00445.html
> > > 
> >
> > Indeed, [2] makes it work again.
> >
> > But taking away IO_MEM_ROM really looks like a lazy workaround. I don't
> > know how much Seabios needs to write - can't it use normal RAM for this?
>
> Handling a read-only f-segment in SeaBIOS would result in significant
> code complexity, and some features (like smbios) couldn't be made to
> work at all.
>
> I believe it would be far simpler to add a mechanism for
> locking/unlocking the f-segment in non-pci mode than it would be to
> make seabios work with a read-only f-segment.
>
> Finally, in the normal pci enabled case, both SeaBIOS and bochs bios
> will lock the f-segment regardless of how it starts.  So, the patch at
> [1] is pretty harmless and it leads to an overall simplification.

On the other hand the F-segment will stay unlocked on isapc.

Bochs BIOS comes in two flavours: BIOS-bochs-legacy (rombios) and
BIOS-bochs-latest (rombios + rombios32). None of both requires the
F-segment to be writeable. BIOS-bochs-legacy doesn't put anything there.
BIOS-bochs-latest puts some tables (e.g. mp, acpi) there unless
BX_USE_EBDA_TABLES is set.

- Sebastian