It depends. On cortex-m, it is the CPU. On RV, it's the code. However, if your IRQs are within the ABI, you are forced to save some registers. I believe Xilinx

It depends. On cortex-m, it is the CPU. On RV, it's the code. However, if your IRQs are within the ABI, you are forced to save some registers. I believe Xilinx is overly aggressive with their saving but you still need to save at least some without hardware assistance.