arch-arm: ISA param for treating MISCREG_IMPDEF_UNIMPL as NOP

In the Arm ISA there are some sys reg numbers which are reserved for
implementation defined registers. The default behaviour is to to treat
them as unimplemented registers. It is now possible to change this
behaviour at runtime and treat them as NOP. In this way an access to
those register won't make simulation fail.

Change-Id: I0d108299a6d5aa81fcdabdaef04eafe46df92343
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10504
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

src/arch/arm/ArmISA.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2012-2013, 2015-2016 ARM Limited
+# Copyright (c) 2012-2013, 2015-2016, 2018 ARM Limited
 # All rights reserved.
 #
 # The license below extends only to copyright in the software and shall
@@ -111,3 +111,9 @@ class ArmISA(SimObject):
     # Reserved for future expansion
     id_aa64mmfr1_el1 = Param.UInt64(0x0000000000000000,
         "AArch64 Memory Model Feature Register 1")
+
+    # Any access (read/write) to an unimplemented
+    # Implementation Defined registers is not causing an Undefined Instruction.
+    # It is rather executed as a NOP.
+    impdef_nop = Param.Bool(False,
+        "Any access to a MISCREG_IMPDEF_UNIMPL register is executed as NOP")

src/arch/arm/isa.cc

@@ -61,7 +61,8 @@ ISA::ISA(Params *p)
       system(NULL),
       _decoderFlavour(p->decoderFlavour),
       _vecRegRenameMode(p->vecRegRenameMode),
-      pmu(p->pmu)
+      pmu(p->pmu),
+      impdefAsNop(p->impdef_nop)
 {
     miscRegs[MISCREG_SCTLR_RST] = 0;
 

src/arch/arm/isa.hh

@@ -90,6 +90,12 @@ namespace ArmISA
         bool haveLargeAsid64;
         uint8_t physAddrRange64;
 
+        /**
+         * If true, accesses to IMPLEMENTATION DEFINED registers are treated
+         * as NOP hence not causing UNDEFINED INSTRUCTION.
+         */
+        bool impdefAsNop;
+
         /** MiscReg metadata **/
         struct MiscRegLUTEntry {
             uint32_t lower;  // Lower half mapped to this register

src/arch/arm/isa/formats/aarch64.isa

@@ -377,6 +377,20 @@ namespace Aarch64
                         return new FailUnimplemented(read ? "mrs" : "msr",
                             machInst, full_mnemonic);
 
+                    } else if (miscReg == MISCREG_IMPDEF_UNIMPL) {
+                        auto full_mnemonic =
+                            csprintf("%s op0:%d op1:%d crn:%d crm:%d op2:%d",
+                                     read ? "mrs" : "msr",
+                                     op0, op1, crn, crm, op2);
+
+                        if (miscRegInfo[miscReg][MISCREG_WARN_NOT_FAIL]) {
+                            return new WarnUnimplemented(read ? "mrs" : "msr",
+                                machInst, full_mnemonic + " treated as NOP");
+                        } else {
+                            return new FailUnimplemented(read ? "mrs" : "msr",
+                                machInst, full_mnemonic);
+                        }
+
                     } else if (miscRegInfo[miscReg][MISCREG_IMPLEMENTED]) {
                         if (miscReg == MISCREG_NZCV) {
                             if (read)

src/arch/arm/isa/formats/misc.isa

@@ -220,10 +220,22 @@ let {{
                     csprintf("miscreg crn:%d opc1:%d crm:%d opc2:%d %s unknown",
                     crn, opc1, crm, opc2, isRead ? "read" : "write"));
           case MISCREG_IMPDEF_UNIMPL:
-            return new McrMrcImplDefined(
-                isRead ? "mrc implementation defined" :
-                         "mcr implementation defined",
-                machInst, iss, MISCREG_IMPDEF_UNIMPL);
+
+            if (miscRegInfo[miscReg][MISCREG_WARN_NOT_FAIL]) {
+                auto mnemonic =
+                    csprintf("miscreg crn:%d opc1:%d crm:%d opc2:%d %s",
+                             crn, opc1, crm, opc2, isRead ? "read" : "write");
+
+                return new WarnUnimplemented(
+                    isRead ? "mrc implementation defined" :
+                             "mcr implementation defined",
+                    machInst, mnemonic + " treated as NOP");
+            } else {
+                return new McrMrcImplDefined(
+                    isRead ? "mrc implementation defined" :
+                             "mcr implementation defined",
+                    machInst, iss, MISCREG_IMPDEF_UNIMPL);
+            }
           case MISCREG_CP15ISB:
             return new Isb(machInst, iss);
           case MISCREG_CP15DSB:

src/arch/arm/miscregs.cc

@@ -4008,6 +4008,9 @@ ISA::initializeMiscRegMetadata()
       .unimplemented()
       .warnNotFail();
     InitReg(MISCREG_UNKNOWN);
+    InitReg(MISCREG_IMPDEF_UNIMPL)
+      .unimplemented()
+      .warnNotFail(impdefAsNop);
 
     // Register mappings for some unimplemented registers:
     // ESR_EL1 -> DFSR