[m5-users] Storing thread/process context for resuming later on

[Nicolas Zea]

I'm trying to implement a version of context switching that involves  
storing the context of a cpu from one point in time in a hardware  
storage, then at a later point in time reloading that context,  
possibly on a different cpu. Previously I had done something similar,  
where the benchmark on core 1 was restarted on core 2. My method for  
this was backing up the AlphaLiveProcess ptr until I want to resume  
that process. At that point I'd start a drain event, clear the  
registers, attach the process to the cpu and replace the process'  
thread context with that from the cpu, call startup on the process,  
then resume the cpu.

The following code did this (after the drain completed):
                // reset the registers in cpu (to 0)
                for(int i=0; i<thecpu->regFile.numPhysicalIntRegs; i++){
                        thecpu->regFile.intRegFile[i] = 0;
                }
                for(int i=0; i<thecpu->regFile.numPhysicalFloatRegs; i++){
                        thecpu->regFile.floatRegFile[i].d = 0;
                }
                for(int i=0; i<AlphaSimpleImpl::MaxThreads; i++){
                        thecpu->regFile.miscRegs[i].clear();
                }

                if(pkt->servo_proc->pTable != NULL){
                        // reset the page table
                        delete myproc->pTable;
                        myproc->pTable = new PageTable(myproc->system);
                }
                
                // make sure to reset the translating port
		if(((AlphaTC<AlphaSimpleImpl> *)(thecpu->threadContexts[0]))->thread- 
>port != NULL){
                        // reset the translating port so it uses the new page 
table
			delete ((AlphaTC<AlphaSimpleImpl> *)(thecpu->threadContexts[0]))- 
>thread->port;
			((AlphaTC<AlphaSimpleImpl> *)(thecpu->threadContexts[0]))->thread- 
>port = NULL;
                }
                                
		((AlphaTC<AlphaSimpleImpl> *)(thecpu->threadContexts[0]))->thread- 
>process = pkt->servo_proc;  // attach the process		
                thecpu->registerThreadContexts();    // register the process
                                
		myproc->replaceThreadContext(thecpu->threadContexts[0], 0); 	//  
attach the thread to the process

		myproc->startup();	// perform the initial write of program data into  
memory, and reset the cpu state (PC, NPC, SP, etc.)	

What I'd like do now is resume from the exact point the process was  
executing before. I've backed up the register values, as well as PC  
and NPC in an data container. I assumed that I could simply use the  
same steps, except instead of using myproc->startup() to set the PC,  
I'd set it manually (thecpu->setPC()). Additionally, I wouldn't reset  
the page table, as it would be reused.

This doesn't appear to work though, as the PC does not appear to get  
reloaded from commit.PC (which is what I gather setPC modifies),  
unless I call fetch.takeOverFrom(), at which point other errors in the  
different pipeline stages arise. Does anyone have some advice on the  
steps required to do this kind of contextual switch?

For information, I'm running this on m5 2.0b2 in ALPHA_SE mode, and  
all the processes are independent single-threaded applications.

-Nick
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