MMIO emulation can fail if the guest uses an instruction that we are
not prepared to emulate. Since these instructions can be and most
likely are valid ones, this is (slightly) closer to an access fault
than to an illegal instruction, so deliver a Data Storage interrupt
instead of a Program interrupt.

BookE ignores bad faults, so it will keep using a Program interrupt
because a DSI would cause a fault loop in the guest.
Suggested-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220125215655.1026224-6-farosas@linux.ibm.com