Function handle_mret

crates/virtual-machine/src/cpu/traps.rs:145–168

Handle MRET - return from machine-mode trap. Restores MIE from MPIE, privilege from MPP, returns PC from MEPC.

(regs: &mut Registers, csrs: &mut CsrFile)

/// Handle SRET - restore SIE from SPIE, privilege from SPP, return PC from SEPC. Err if illegal.
pub fn handle_sret(regs: &mut Registers, csrs: &mut CsrFile) -> Result<u64, VmError> {
    // SRET is illegal from U-mode.
    if regs.priv_mode == PrivilegeMode::User {
        return Err(VmError::IllegalInstruction(0x1020_0073));
    }
    // TSR bit (mstatus[22]): if set while in S-mode, SRET raises illegal instruction.
    if regs.priv_mode == PrivilegeMode::Supervisor && (csrs.mstatus >> 22) & 1 == 1 {
        return Err(VmError::IllegalInstruction(0x1020_0073));
    }

    // SIE = old SPIE; SPIE = 1
    let spie = (csrs.mstatus >> 5) & 1;
    csrs.mstatus = (csrs.mstatus & !(1u64 << 1)) | (spie << 1); // SIE = old SPIE
    csrs.mstatus |= 1u64 << 5; // SPIE = 1

    // Restore privilege from SPP (mstatus[8]), then set SPP = User (0).
    let spp = (csrs.mstatus >> 8) & 1;
    let prev_priv = if spp == 0 {
        PrivilegeMode::User
    } else {
        PrivilegeMode::Supervisor
    };
    regs.priv_mode = prev_priv;
    csrs.mstatus &= !(1u64 << 8); // SPP = User (0)

    // Clear MPRV (not returning to M-mode).
    csrs.mstatus &= !(1u64 << 17);