While booting a 64bit kernel, there is a small timeframe where OF and the kernel communicate with each other. Within that timeframe, DSI/ISI interrupts may occur, because some memory is not mapped yet.
Right now in case that happens, we jump into the DSI/ISI interrupt handler which clobbers the high 32 bits of the kernel's registers. In order to circumvent that, let's save/restore all 64 bits of all kernel registers when we get a DSI/ISI interrupt.
This patch enables a PPC64 Linux kernel to boot up to the point where it tries to set up the SLB entries (slbmte), which is not yet implemented in qemu.
v2 implements handling for the HIOR register, bringing interrupt handlers to RAM.