Fix mret and mnret instructions (#588)

* Fix mret and mnret instructions to support correctly Smdbltrp (MDT bit) and MPRV bit
Before his fix IDL code reset mstatus.MPRV bit only in case if S mode
is implemented for both mret and mnret. This seems wrong, since spec
states that MPRV should be reset by both mret and mnret for case when
mode processor is returning is different from Machine mode (2b'11).
As well, Smdbltrp spec requires that such case (returning to non-machine
mode), the mnret instruction should reset MDT bit.
According to the spec mnret instruction should set mnstatus.NMIE=1.
Also, mnret should use mnstatus.MNPP bit field for previous mode
storage.

Signed-off-by: Albert Yosher <quic_ayosher@quicinc.com>

* MNRET instruction: improving long name and description to match Smrnmi and Smdbltrp extensions documentation

Signed-off-by: Albert Yosher <quic_ayosher@quicinc.com>

---------

Signed-off-by: Albert Yosher <quic_ayosher@quicinc.com>
Co-authored-by: Derek Hower <134728312+dhower-qc@users.noreply.github.com>

Author: ayosher

arch/inst/I/mret.yaml

@@ -14,9 +14,16 @@ access:
 encoding:
   match: "00110000001000000000000001110011"
 operation(): |
-  if (implemented?(ExtensionName::S) && CSR[mstatus].MPP != 2'b11) {
+  if (CSR[mstatus].MPP != 2'b11) {
     CSR[mstatus].MPRV = 0;
   }
+  if (implemented?(ExtensionName::Smdbltrp)) {
+    if (xlen() == 64) {
+      CSR[mstatus].MDT = 1'b0;
+    } else {
+      CSR[mstatush].MDT = 1'b0;
+    }
+  }
   CSR[mstatus].MIE = CSR[mstatus].MPIE;
   CSR[mstatus].MPIE = 1;
   if (CSR[mstatus].MPP == 2'b00) {

arch/inst/Smrnmi/mnret.yaml

@@ -3,9 +3,13 @@
 $schema: inst_schema.json#
 kind: instruction
 name: mnret
-long_name: No synopsis available.
+long_name: Machine mode resume from the RNMI or Double Trap handler.
 description: |
-  No description available.
+  MNRET is an M-mode-only instruction that uses the values in mnepc and mnstatus to return to the
+  program counter, privilege mode, and virtualization mode of the interrupted context. This instruction
+  also sets mnstatus.NMIE. If MNRET changes the privilege mode to a mode less privileged than M, it
+  also sets mstatus.MPRV to 0. If the Zicfilp extension is implemented, then if the new privileged mode is
+  y, MNRET sets ELP to the logical AND of yLPE (see Section 22.1.1) and mnstatus.MNPELP.
 definedBy: Smrnmi
 assembly: mnret
 encoding:
@@ -18,9 +22,17 @@ access:
   vu: always
 data_independent_timing: false
 operation(): |
-  if (implemented?(ExtensionName::S) && CSR[mstatus].MPP != 2'b11) {
+  if (CSR[mnstatus].MNPP != 2'b11) {
     CSR[mstatus].MPRV = 0;
+    if (implemented?(ExtensionName::Smdbltrp)) {
+      if (xlen() == 64) {
+        CSR[mstatus].MDT = 1'b0;
+      } else {
+        CSR[mstatush].MDT = 1'b0;
+      }
+    }
   }
+  CSR[mnstatus].NMIE = 1'b1;
   if (CSR[mnstatus].MNPP == 2'b00) {
     set_mode(PrivilegeMode::U);
   } else if (CSR[mnstatus].MNPP == 2'b01) {