Implement elp state holding mechanism for mnstatus

riscv/csrs.cc

@@ -585,6 +585,7 @@ bool mnstatus_csr_t::unlogged_write(const reg_t val) noexcept {
   // NMIE can be set but not cleared
   const reg_t mask = (~read() & MNSTATUS_NMIE)
                    | (proc->extension_enabled('H') ? MNSTATUS_MNPV : 0)
+                   | (proc->extension_enabled(EXT_ZICFILP) ? MNSTATUS_MNPELP : 0)
                    | MNSTATUS_MNPP;
 
   const reg_t requested_mnpp = proc->legalize_privilege(get_field(val, MNSTATUS_MNPP));

riscv/insns/mnret.h

@@ -10,5 +10,11 @@ if (prev_prv != PRV_M) {
   STATE.mstatus->write(mstatus);
 }
 s = set_field(s, MNSTATUS_NMIE, 1);
+if (ZICFILP_xLPE(prev_virt, prev_prv)) {
+  STATE.elp = static_cast<elp_t>(get_field(s, MNSTATUS_MNPELP));
+}
+if (p->extension_enabled(EXT_ZICFILP)) {
+  s = set_field(s, MNSTATUS_MNPELP, elp_t::NO_LP_EXPECTED);
+}
 STATE.mnstatus->write(s);
 p->set_privilege(prev_prv, prev_virt);