Using OFW for Pegasos2 emulation and as generic PPC firmware in QEMU
by BALATON Zoltan
Hello,
I'm trying to use OFW for the Pegasos2 emulation in QEMU that I'm working
on but at the same time I'm trying to make this generic with the promise
that it could later be used for other machines emulated by QEMU as well
(such as 40p and Macs and maybe even sparc machines but for those
compiling the OBP sources may be better as these two seem to have diverged
too much, so this is only for PPC now and mainly focusing on Pegasos2 at
first). Starting from Artyom's prep/qemu port I've got something starting
but it's not working yet and I have several questions and need some advice
on how to continue.
My current work in progress code is here:
https://github.com/zbalaton/openfirmware/tree/master/cpu/ppc/qemu
that can be tested with the currently off-tree pegasos2 emulation from:
https://osdn.net/projects/qmiga/scm/git/qemu/tree/pegasos2/
(this is QEMU with WIP pegasos2 patch) only needed to compile with
configure --target-list=ppc-softmmu to get qemu-system-ppc with pegasos2
machine, then it can be tested with:
qemu-system-ppc -M pegasos2 -m 512 -bios qofwppc.rom -serial stdio
where qofwppc.rom is what's built from the above OFW fork. More info on
the Pegasos2 emulation can be found here:
https://osdn.net/projects/qmiga/wiki/SubprojectPegasos2
To keep it generic and easy to adapt in the future to other machines I'm
trying to do what SLOF does and use a flattened DTB to pass initial
parameters from QEMU to OFW and start from there instead of detecting
hardware as it would be done on a real machine. (Since this is only for
QEMU emulated machines, a lot of low level hardware init and testing that
OFW would normally do on real hadrware can be skipped as those are not
needed for a virtual machine.) I've attempted to port SLOF's fdt parsing,
which is in fdt.fth. It's able to create the initial device tree but may
have some problems so it needs to be reviewed by someone who knows OFW.
The rest is mostly a cut down version of cpu/ppc/prep/qemu with as much
machine specific parts removed as I could. The idea I've tried was to put
machine specifics in a drop-in (so a generic rom could be built for
similar machines that would load the appropriate driver based on the
device tree it got from QEMU or it would be easy to build different roms
for different machines with common parts still being the same). I only
have one such driver now: the pegasos2.bth implements that drop-in but
some code I could not put in there as those do not seem to work in fc file
so those are now in pegasos2.fth. Ideally these should also be in the
machine specific driver but I'm not sure that's possible. If not then
maybe the above idea can't work and we'll need separate rom image for each
machine but maybe there are some Forth tricks I'm not aware of to make
this work.
Currently this gets to an OFW prompt but detecting pci devices does not
work yet. With the default (for pegasos2) cirrus vga a /pci/vga device
node is created but I think it stops during that and does not detect any
more pci devices. With -vga std (that's the normal QEMU Bochs compatible
VGA) it freezes during init-ing the card and with my experimental -vga
none -device ati-vga (that's also upstream in QEMU now) an unnamed node is
created so something is wrong with PCI probing at the moment. Could it be
because not properly doing memory scrub&release? The the prep/qemu rom has
some code for this and SLOF fdt code also has some parts to handle that
but I did not understand how this should work so I've disabled these but
it could be this causes problems with memory management later?
Once this can be made working and would get PCI devices detected next
question is if it would be possible to emulate the board's original
firmware that the OSes running on it expect. It's an OpenFirmware
implementation (SmartFirmware) but properties may be slightly different so
it may be necessary to change some properties to match that but I'm not
sure what's the best way to do that.
Lastly once Pegasos2 works I may be interested in trying to make this work
with the emulated Mac machines in QEMU as well but it seems drivers for
that would be needed as those are not included in the OFW sources so it
may be a bigger task so for now I focus on Pegasos2 but keeping it open
for future enhancement.
I think I need more understanding on how OFW works and if the above is
possible at all or how to debug it further so I hope someone here can
provide some insight or answer my questions to get me over these hurdles.
(I've already read http://wiki.laptop.org/go/Forth_Lessons which was a
great introduction also discussing OFW implementation details but that
still left me with a lot of pieces missing so I'd need some more help.)
Thank you,
BALATON Zoltan
6 months, 3 weeks
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