Got Insutrction Fetch Exception on new QEMU version

[timlee0212]

The code starting from Chapter 3 does not work on newer versions of QEMU (it seems for any version >= 6.0). You will get an exception as soon as entering the supervisor mode with mcause=1 . This indicates an instruction access fault as per the manual.

After some investigation, I think this might be caused by the newer version of QEMU implementing Physical Memory Protection (PMP). As mentioned in the manual, if the registers are not configured, all memory access in supervisor mode will fail.

A simple (but dirty) solution is to bypass the PMP by adding one configuration in the kinit function.

        asm!("csrw pmpaddr0, {}", in(reg) 0);
        asm!("csrw pmpcfg0, {}", in(reg) (7 | (1 << 3))); // RWX, A_TOR

This allows read, write, and execution for all physical addresses starting from 0.

Reference: https://five-embeddev.com/riscv-isa-manual/latest/machine.html#sec:pmp