arch-arm: Fix SoftwareStep::debugExceptionReturnSS

debugExceptionReturnSS is called on an ERET instruction to
check for software step. The method was not using the
SPSR.width and it was relying on the more generic ELIs32 to
check the execution mode of the destination EL.

This is not only an efficiency problem: the helper might not work
when returning to EL0. In general it is not possible to
understand if EL0 is using AArch32 or AArch64 if the current
EL is not EL0 and EL1 is using AArch64.

This is instead visible by inspecting the spsr.width during the
execution of an ERET instruction

Change-Id: Ibc5a43633d0020139f2c0e372959a3ab4880da6e
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32634
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/arm/insts/static_inst.cc

@@ -1194,7 +1194,7 @@ ArmStaticInst::getPSTATEFromPSR(ThreadContext *tc, CPSR cpsr, CPSR spsr) const
 
     SelfDebug *sd = ArmISA::ISA::getSelfDebug(tc);
     SoftwareStep *ss = sd->getSstep();
-    new_cpsr.ss = ss->debugExceptionReturnSS(tc, spsr, dest, new_cpsr.width);
+    new_cpsr.ss = ss->debugExceptionReturnSS(tc, spsr, dest);
 
     return new_cpsr;
 }

src/arch/arm/self_debug.cc

@@ -643,7 +643,7 @@ WatchPoint::compareAddress(ThreadContext *tc, Addr in_addr, uint8_t bas,
 
 bool
 SoftwareStep::debugExceptionReturnSS(ThreadContext *tc, CPSR spsr,
-                                     ExceptionLevel dest, bool aarch32)
+                                     ExceptionLevel dest)
 {
     bool SS_bit = false;
     bool enabled_src = false;
@@ -652,9 +652,7 @@ SoftwareStep::debugExceptionReturnSS(ThreadContext *tc, CPSR spsr,
 
         bool enabled_dst = false;
         bool secure = isSecureBelowEL3(tc) || dest == EL3;
-//        CPSR cpsr = tc->readMiscReg(MISCREG_CPSR);
-//        if (cpsr.width) {
-        if (ELIs32(tc, dest)) {
+        if (spsr.width) {
             enabled_dst = conf->isDebugEnabledForEL32(tc, dest, secure,
                                                       spsr.d == 1);
         } else {

src/arch/arm/self_debug.hh

@@ -210,7 +210,7 @@ class SoftwareStep
     {}
 
     bool debugExceptionReturnSS(ThreadContext *tc, CPSR spsr,
-                                ExceptionLevel dest, bool aarch32);
+                                ExceptionLevel dest);
     bool advanceSS(ThreadContext *tc);
 
     inline void