see soc/fu/trap/main_stage.py trap() function, and:

see soc/fu/trap/main_stage.py trap() function, and:
https://libre-soc.org/irclog/%23libre-soc.2022-01-18.log.html#t2022-01-18T13:21:25
bits of SRR1 need to be preserved on an interrupt, which means that
PowerDecoder2 must schedule a read of SRR1.  the Power ISA spec
is extremely obscure and obtuse on which bits must be preserved,
therefore it is just easier to copy microwatt behaviour