Re: [Rpcemu] RPCEmu licence and other topics

On Wed, Aug 25, 2021 at 08:06:05AM +0000, Sarah wrote:
> On Wednesday, 25 August 2021, 08:56:07 BST, David Feugey <dfeugey@ascinfo.fr> wrote:
> >The other way would be also to correct Qemu to provide a better Raspberry Pi emulation.
>
> This is by far the correct option. Apply the fix where the bug is - if RO
> is running correctly on a real Pi, then bugs need fixing in QEMU's Pi
> emulation. I'm sure the QEMU maintainers would be very happy for any
> patches to improve things on their end.

The raspi2 emulation in QEMU is not a full emulation - it emulates just
about enough to get Linux going. The reason is that the Pi is not just
hardware - a large part of the platform is the GPU firmware providing
services to the ARM-side OS (through mailboxes). These services have
evolved over time - the firmware is part of the SD card image, so you have
to synchronise your OS with the API provided by your firmware.

While it is possible to extend QEMU to implement more firmware services,
it's a perpetual game of catchup and you risk affecting your ability to boot
older/newer Linux/BSD/etc images.

RISC OS currently uses some undocumented firmware interfaces - the
particular one I found is easy to change in RISC OS to use a documented
interface which is hopefully more stable.

Further down the line it is arguable whether RISC OS should be modified to
be less tightly fitted to Pi-specific peculiarities, or QEMU modified to
implement more Pi peculiarities.

These are not bugs, but more questions about emulation philosophy. They are
also ones of development process - how easy is it to get updates into RISC
OS or QEMU, how long do they take until they trickle through to the places
people install them from, how to test it doesn't cause breakage with other
OSes QEMU might boot. IMHO I view the RISC OS development process as
relatively lightweight, whereas upstreaming patches into QEMU and
distributions potentially less so.

More generally, better performance can be achieved by the emulation
departing from exactly modelling the hardware - emulating every single
register in a complicated I/O chip is a lot more work than an interface like
HostFS where you slim it down to just what you need. So it may be something
'good enough' to boot RISC OS is better than a perfect emulation.

Theo

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