PowerPC's Virtual Open Firmware uses an unsupported instruction in older CPUs
The PowerPC machines in QEMU have support for Open Firmware in two different implementations: SLOF and VOF. VOF is part of the QEMU source code. It uses a hypercall instruction to get out of the guest and communicate with QEMU.
Among the machines that support VOF usage, the pegasos2 machine can be used along with older CPUs (G3, G4) that don't support the system call instruction with lev=1. So we shouldn't rely on sc 1
in that machine.
We're keeping support for sc 1
with the G3 and G4 for now so there's currently nothing broken. But this should be fixed. The alternative is to replace the hypercall-based mechanism in VOF with an MMIO-based one. So have a specific MMIO address that VOF can write to and would cause an exit to QEMU.