cpu-minor: this is a bug fix for MinorCPU for thread cloning.

cpu-minor: this is a bug fix for MinorCPU for thread cloning.

Inside the code of cloneFunc(…)  //syscall_emul.hh

    cp->initState();  //line 1483
    p->clone(tc, ctc, cp, flags);  //line 1484
    …
    ctc->clearArchRegs(); //line 1503

    OS::archClone(flags, p, cp, tc, ctc, newStack, tlsPtr); //line 1505
    …

At line 1483, initState() is called and the activateContext() of the
corresponding MinorCPU is eventually called. The actual architecture
clone happens at line 1505 where PC of the new thread could have a
correct value.

In the existing implementation of MinorCPU::activateContext(ThreadID
thread_id), the below line 275 is called
    pipeline->wakeupFetch(thread_id);
to start fetching instruction with current value of PC, which is 0x0,
leading to panic “Page table fault when accessing virtual address 0”.

This is because the OS::archClone() is not yet called. So, the below bug
fix handles the wakeup fetch for a thread for two scenarios:
   ...
    if (!threads[thread_id]->getUseForClone())
    { //the thread is not cloned
        pipeline->wakeupFetch(thread_id);
    } else {//the thread from clone
        if (fetchEventWrapper != NULL)
            delete fetchEventWrapper;
        fetchEventWrapper = new EventFunctionWrapper([this, thread_id]
          {pipeline->wakeupFetch(thread_id);}, "wakeupFetch");
        schedule(*fetchEventWrapper, clockEdge(Cycles(0)));
    }
    ...
If a thread is not cloned, pipeline->wakeupFetch() is called
immediately.
For the cloned thread, the above bug fix delays the execution of
    pipeline->wakeupFetch()
after the OS::archClone is done. ThreadContext::getUseForClone() return
true if a thread is cloned.

A member variable fetchEventWrapper is added to MinorCPU class for
delayed fetch event.

A member variable useForClone and its corresponding get/set methods are
added to ThreadContext class. This approach allows future reuse of this
useForClone variable by other CPU models if needed and also avoid lots
of changes resulted by modifying parameters of activateContext () and
activate() which are defined as override.

Inside the syscall cloneFunc, the useForClone member of a ThreadContext
object is set via its set method right before Process's initState() is
called, shown as below.
    ctc->setUseForClone(true);
    cp->initState();
    p->clone(tc, ctc, cp, flags);

A few previously failed RISC-V ASM tests have been open in tests.py file
after the bug fix works.

JIRA issue: https://gem5.atlassian.net/browse/GEM5-374

Change-Id: Ibffe46522e2617443d29f49df180692c54830f14
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37315
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>