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

From: Sebastian Herbszt
Subject: [Qemu-devel] Re: Seabios dislikes -M isapc
Date: Tue, 9 Feb 2010 21:47:25 +0100

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

- Sebastian

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