arch-riscv: Make WFI halt the hart

First, this CL modifies the implementation of WFI so it actually put the
calling CPU into sleep.

This CL also adds an id in the IRQ table to represent NMI. This is
because the wakeup path is only implemented on cpu's postInterrupt
function, and it expects an int_num.

We still keep the MISCREG for nmie and nmip instead of merging them into
other ip/ie as that will give the user ability to get/set the nmi
status, which is pretty dangerous.

Change-Id: Idf8a5748990efa20aa9372efa97d3bed2aac82d9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/61511
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/riscv/faults.hh

@@ -93,7 +93,11 @@ enum ExceptionCode : uint64_t
     INT_EXT_USER = 8,
     INT_EXT_SUPER = 9,
     INT_EXT_MACHINE = 11,
-    NumInterruptTypes
+    NumInterruptTypes,
+    // INT_NMI does not exist in the spec, it's a modeling artifact for NMI. We
+    // intentionally set it to be NumInterruptTypes so it can never conflict
+    // with any real INT_NUM in used.
+    INT_NMI = NumInterruptTypes,
 };
 
 enum class FaultType

src/arch/riscv/interrupts.hh

@@ -141,14 +141,22 @@ class Interrupts : public BaseInterrupts
     post(int int_num, int index)
     {
         DPRINTF(Interrupt, "Interrupt %d:%d posted\n", int_num, index);
-        ip[int_num] = true;
+        if (int_num != INT_NMI) {
+            ip[int_num] = true;
+        } else {
+            postNMI();
+        }
     }
 
     void
     clear(int int_num, int index)
     {
         DPRINTF(Interrupt, "Interrupt %d:%d cleared\n", int_num, index);
-        ip[int_num] = false;
+        if (int_num != INT_NMI) {
+            ip[int_num] = false;
+        } else {
+            clearNMI();
+        }
     }
 
     void postNMI() { tc->setMiscReg(MISCREG_NMIP, 1); }

src/arch/riscv/isa/decoder.isa

@@ -1906,7 +1906,7 @@ decode QUADRANT default Unknown::unknown() {
                                             "wfi in user mode or TW enabled",
                                             machInst);
                             }
-                            // don't do anything for now
+                            xc->tcBase()->quiesce();
                         }}, No_OpClass);
                     }
                     0x9: sfence_vma({{