arch-arm: Add misc_accessor templated functions to read/write regs at different ELs (#1072)

A usual system register read/write pattern is something like the
following

```
switch(el) {
    case EL1:
        tc->readMiscReg(REG_EL1);
    case EL2:
        tc->readMiscReg(REG_EL2);
    case EL3:
        tc->readMiscReg(REG_EL3);
}
```

To avoid repeating these switch statements all over gem5, we define
templated functions which have
an accessor struct as a template parameter. These accessor will help
populating the templated switch
construct. We provide the FAR register accessor as an example. The
accessor should define the following
fields: (type, el0, el1, el2, el3)

Example:

```
struct FarAccessor
{
    using type = RegVal;
    static const MiscRegIndex el0 = NUM_MISCREGS;
    static const MiscRegIndex el1 = MISCREG_FAR_EL1;
    static const MiscRegIndex el2 = MISCREG_FAR_EL2;
    static const MiscRegIndex el3 = MISCREG_FAR_EL3;
};
```

src/arch/arm/faults.cc

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2010, 2012-2014, 2016-2019, 2022 Arm Limited
+ * Copyright (c) 2010, 2012-2014, 2016-2019, 2022, 2024 Arm Limited
  * All rights reserved
  *
  * The license below extends only to copyright in the software and shall
@@ -44,6 +44,7 @@
 #include "arch/arm/insts/static_inst.hh"
 #include "arch/arm/interrupts.hh"
 #include "arch/arm/isa.hh"
+#include "arch/arm/regs/misc_accessors.hh"
 #include "arch/arm/self_debug.hh"
 #include "arch/arm/system.hh"
 #include "arch/arm/utility.hh"
@@ -378,22 +379,6 @@ ArmFault::getSyndromeReg64() const
     }
 }
 
-MiscRegIndex
-ArmFault::getFaultAddrReg64() const
-{
-    switch (toEL) {
-      case EL1:
-        return MISCREG_FAR_EL1;
-      case EL2:
-        return MISCREG_FAR_EL2;
-      case EL3:
-        return MISCREG_FAR_EL3;
-      default:
-        panic("Invalid exception level");
-        break;
-    }
-}
-
 void
 ArmFault::setSyndrome(ThreadContext *tc, MiscRegIndex syndrome_reg)
 {
@@ -1113,13 +1098,15 @@ AbortFault<T>::invoke(ThreadContext *tc, const StaticInstPtr &inst)
         if (stage2) {
             // stage 2 fault, set HPFAR_EL2 to the faulting IPA
             // and FAR_EL2 to the Original VA
-            tc->setMiscReg(AbortFault<T>::getFaultAddrReg64(), OVAddr);
+            misc_regs::writeRegister<misc_regs::FarAccessor>(
+                tc, OVAddr, this->toEL);
             tc->setMiscReg(MISCREG_HPFAR_EL2, bits(faultAddr, 47, 12) << 4);
 
             DPRINTF(Faults, "Abort Fault (Stage 2) VA: 0x%x IPA: 0x%x\n",
                     OVAddr, faultAddr);
         } else {
-            tc->setMiscReg(AbortFault<T>::getFaultAddrReg64(), faultAddr);
+            misc_regs::writeRegister<misc_regs::FarAccessor>(
+                tc, faultAddr, this->toEL);
         }
     }
 }
@@ -1517,7 +1504,7 @@ PCAlignmentFault::invoke(ThreadContext *tc, const StaticInstPtr &inst)
     ArmFaultVals<PCAlignmentFault>::invoke(tc, inst);
     assert(from64);
     // Set the FAR
-    tc->setMiscReg(getFaultAddrReg64(), faultPC);
+    misc_regs::writeRegister<misc_regs::FarAccessor>(tc, faultPC, toEL);
 }
 
 bool
@@ -1661,8 +1648,7 @@ Watchpoint::invoke(ThreadContext *tc, const StaticInstPtr &inst)
 {
     ArmFaultVals<Watchpoint>::invoke(tc, inst);
     // Set the FAR
-    tc->setMiscReg(getFaultAddrReg64(), vAddr);
+    misc_regs::writeRegister<misc_regs::FarAccessor>(tc, vAddr, toEL);
-
 }
 
 bool

src/arch/arm/faults.hh

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2010, 2012-2013, 2016-2019, 2022 Arm Limited
+ * Copyright (c) 2010, 2012-2013, 2016-2019, 2022, 2024 Arm Limited
  * All rights reserved
  *
  * The license below extends only to copyright in the software and shall
@@ -220,9 +220,6 @@ class ArmFault : public FaultBase
     // Returns the actual syndrome register to use based on the target
     // exception level
     MiscRegIndex getSyndromeReg64() const;
-    // Returns the actual fault address register to use based on the target
-    // exception level
-    MiscRegIndex getFaultAddrReg64() const;
 
     void invoke(ThreadContext *tc, const StaticInstPtr &inst =
                 nullStaticInstPtr) override;

src/arch/arm/regs/misc_accessors.hh

@@ -0,0 +1,105 @@
+/*
+ * Copyright (c) 2024 Arm Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef __ARCH_ARM_REGS_MISC_ACCESSORS_HH__
+#define __ARCH_ARM_REGS_MISC_ACCESSORS_HH__
+
+#include "arch/arm/regs/misc_types.hh"
+#include "cpu/thread_context.hh"
+
+namespace gem5
+{
+
+namespace ArmISA
+{
+
+namespace misc_regs
+{
+
+struct FarAccessor
+{
+    using type = RegVal;
+    static const MiscRegIndex el0 = NUM_MISCREGS;
+    static const MiscRegIndex el1 = MISCREG_FAR_EL1;
+    static const MiscRegIndex el2 = MISCREG_FAR_EL2;
+    static const MiscRegIndex el3 = MISCREG_FAR_EL3;
+};
+
+template <typename RegAccessor>
+MiscRegIndex
+getRegVersion(ExceptionLevel el)
+{
+    switch (el) {
+      case EL0:
+        return RegAccessor::el0;
+      case EL1:
+        return RegAccessor::el1;
+      case EL2:
+        return RegAccessor::el2;
+      case EL3:
+        return RegAccessor::el3;
+      default:
+        panic("Invalid EL\n");
+    }
+}
+
+template <typename RegAccessor>
+typename RegAccessor::type
+readRegister(ThreadContext *tc, ExceptionLevel el)
+{
+    return tc->readMiscReg(getRegVersion<RegAccessor>(el));
+}
+
+template <typename RegAccessor>
+typename RegAccessor::type
+readRegisterNoEffect(ThreadContext *tc, ExceptionLevel el)
+{
+    return tc->readMiscRegNoEffect(getRegVersion<RegAccessor>(el));
+}
+
+template <typename RegAccessor>
+void
+writeRegister(ThreadContext *tc, RegVal val, ExceptionLevel el)
+{
+    tc->setMiscReg(getRegVersion<RegAccessor>(el), val);
+}
+
+} // namespace misc_regs
+} // namespace ArmISA
+} // namespace gem5
+
+#endif // __ARCH_ARM_REGS_MISC_ACCESSORS_HH__