Re: [Qemu-devel] qemu/pc-bios ppc_rom.bin

[Thiemo Seufer <ths@networkno.de>]

Blue Swirl wrote:
> On 10/1/07, Jocelyn Mayer <l_indien@magic.fr> wrote:
> > On Mon, 2007-10-01 at 17:55 +0300, Blue Swirl wrote:
> > > On 10/1/07, Andreas Färber <andreas.faerber@web.de> wrote:
> > > >
> > > > Am 01.10.2007 um 09:12 schrieb Bob Deblier:
> > > >
> > > > > Ideally we should have an OpenBIOS compiled for QEMU/PPC. Is anyone
> > > > > working on this?
> > > >
> > > > I had looked into this recently but it turned out that PearPC and
> > > > others using OpenBIOS/ppc use an ELF format OpenBIOS binary that is
> > > > incompatible with QEMU, expecting some raw image. I have no idea how
> > > > to go about this; the (working) sparc version uses some "weird"
> > > > assembler initializations. ;-)
> > >
> > > You can use:
> > > objcopy -O binary in.elf out.bin
> > >
> > > Alternatively, Qemu could be enhanced to try loading ELF first and
> > > binary if that fails.
> >
> > This is even not an option. With "normal" full system emulation, Qemu
> > boots like real hardware does. I don't know any CPU able to load ELF
> > images. As the goal is to emulate real hardware, what is to be given is
> > a ROM image, able to boot a real machine.

FWIW, I don't regard the on-disk format of the BIOS as essential, as
long as the emulated CPU sees the same bits as a real cpu does.

Accepting ELF as a (possibly alternative) container for a BIOS image
is a matter of convenience.

> The effect is exactly the same from the emulated CPU perspective. With
> ELF image we gain symbols in the out_asm dump.
> 
> > You can try to ehance the -kernel option to do weird hacks if you like
> > but the CPU state at the start of a normal boot process should be as
> > near as possible as a real CPU after a hard reset. Any other behavior is
> > a bug to fix asap.
> > Imho Qemu can be a very great development tool (and I already used it
> > for this purpose), not just a geek toy, then hacks that do not reflect
> > what real hardware does have to be avoided any time it's possible. Then,
> > adding an ELF loader in the CPU initialisation code seems to be a
> > nonsense. The goal to achieve, imho, is to be able to run real ROM
> > images extracted from real machine, not to "extend" the CPU features
> > with stuffs that has no reality (and are even not useful as long as no
> > machine would never accept to boot on this "firmware").
> 
> Qemu is not limited to just hardware emulation. Please consider for
> example snapshot load/save support, built-in gdbstub and monitor. No
> real hardware has any of these, or perhaps you could do similar things
> with ICE or JTAG.
> 
> Qemu is not also aimed for 100% accurate emulation of the hardware.
> There are no caches or cycle counters and hardware devices run
> unrealistically fast from CPU standpoint. Emulating performance
> counters or the errata the most CPUs have would be extremely
> difficult. I doubt Qemu CPU emulation can ever pass POST of real
> BIOSes.

I am working on making the Malta emulation boot a unaltered YAMON
image. I don't see why a PC BIOS would be harder to accomodate.

> Real BIOSes are also closed source, proprietary binary blobs.

At least YAMON, CFE and PMON are not closed source. YAMON has a funny
license which - I hope - will change.

> Making open source BIOSes a viable alternative is in my opinion a much
> more important goal.

The one doesn't exclude the other. That said, I regard the ability to
boot unaltered real-world firmare as an important test of the quality
of a system emulation.


Thiemo