sim: Move findFreeContext to System.

This method searches through the ThreadContexts stored in the system,
and has no concrete connection to Process other than it happened to be
used by a Process in the clone system call.

By moving it, we can use its functionality in classes other than
Process.

Change-Id: Ic6899c335dc13841c6fe948ea3a4f8ad67e562bb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24285
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/process.cc

@@ -280,16 +280,6 @@ Process::regStats()
         ;
 }
 
-ThreadContext *
-Process::findFreeContext()
-{
-    for (auto &it : system->threadContexts) {
-        if (ThreadContext::Halted == it->status())
-            return it;
-    }
-    return nullptr;
-}
-
 void
 Process::revokeThreadContext(int context_id)
 {

src/sim/process.hh

@@ -129,9 +129,6 @@ class Process : public SimObject
         contextIds.push_back(context_id);
     }
 
-    // Find a free context to use
-    ThreadContext *findFreeContext();
-
     /**
      * After delegating a thread context to a child process
      * no longer should relate to the ThreadContext

src/sim/syscall_emul.hh

@@ -1527,7 +1527,7 @@ cloneFunc(SyscallDesc *desc, int callnum, ThreadContext *tc)
         return -EINVAL;
 
     ThreadContext *ctc;
-    if (!(ctc = p->findFreeContext())) {
+    if (!(ctc = tc->getSystemPtr()->findFreeContext())) {
         DPRINTF_SYSCALL(Verbose, "clone: no spare thread context in system"
                         "[cpu %d, thread %d]", tc->cpuId(), tc->threadId());
         return -EAGAIN;

src/sim/system.cc

@@ -302,6 +302,16 @@ System::registerThreadContext(ThreadContext *tc, ContextID assigned)
     return id;
 }
 
+ThreadContext *
+System::findFreeContext()
+{
+    for (auto &it : threadContexts) {
+        if (ThreadContext::Halted == it->status())
+            return it;
+    }
+    return nullptr;
+}
+
 bool
 System::schedule(PCEvent *event)
 {

src/sim/system.hh

@@ -188,6 +188,14 @@ class System : public SimObject, public PCEventScope
     unsigned int cacheLineSize() const { return _cacheLineSize; }
 
     std::vector<ThreadContext *> threadContexts;
+    ThreadContext *findFreeContext();
+
+    ThreadContext *
+    getThreadContext(ContextID tid) const
+    {
+        return threadContexts[tid];
+    }
+
     const bool multiThread;
 
     using SimObject::schedule;
@@ -195,11 +203,6 @@ class System : public SimObject, public PCEventScope
     bool schedule(PCEvent *event) override;
     bool remove(PCEvent *event) override;
 
-    ThreadContext *getThreadContext(ContextID tid) const
-    {
-        return threadContexts[tid];
-    }
-
     unsigned numContexts() const { return threadContexts.size(); }
 
     /** Return number of running (non-halted) thread contexts in