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 <[email protected]>

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 +3,9 @@
 $schema: inst_schema.json#
 kind: instruction
 name: mnret
-long_name: No synopsis available.
+long_name: Machine NMI Return
 description: |
-  No description available.
+  Returns from an NMI in M-mode.
 definedBy: Smrnmi
 assembly: mnret
 encoding:
@@ -18,9 +18,17 @@ 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) {