cpu: Implement basic HTM capabilities in the CheckerCPU

The O3CPU, which supports transactional memory (HTM), is using
the inHtmTransactionalState and getHtmCheckpointPtr methods
to check if we are in the middle of a transaction and return
false or a nullptr if that's not the case.

We need to avoid aborting simulation (panic) when those methods are
called in the O3CPU + Checker simulation.

This patch is providing the minimal support to re-enable O3 + Checker
runs and it is not providing HTM support in the CheckerCPU (meaning, we
won't be able to use the Checker in a transactional simulation)

Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Change-Id: I7f71d5290c53b0402763d69f137ecaa1208253fb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/46624
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/cpu/checker/cpu.hh

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011, 2016-2018, 2020 ARM Limited
+ * Copyright (c) 2011, 2016-2018, 2020-2021 Arm Limited
  * Copyright (c) 2013 Advanced Micro Devices, Inc.
  * All rights reserved
  *
@@ -380,15 +380,14 @@ class CheckerCPU : public BaseCPU, public ExecContext
     bool
     inHtmTransactionalState() const override
     {
-        panic("not yet supported!");
-        return false;
+        return (getHtmTransactionalDepth() > 0);
     }
 
     uint64_t
     getHtmTransactionalDepth() const override
     {
-        panic("not yet supported!");
-        return 0;
+        assert(thread->htmTransactionStarts >= thread->htmTransactionStops);
+        return (thread->htmTransactionStarts - thread->htmTransactionStops);
     }
 
     TheISA::PCState pcState() const override { return thread->pcState(); }

src/cpu/checker/thread_context.hh

@@ -497,7 +497,7 @@ class CheckerThreadContext : public ThreadContext
     BaseHTMCheckpointPtr&
     getHtmCheckpointPtr() override
     {
-        panic("function not implemented");
+        return actualTC->getHtmCheckpointPtr();
     }
 
     void