src/gpu-compute/gpu_tlb.hh - public/gem5 - Git at Google

 /*
  * Copyright (c) 2011-2015 Advanced Micro Devices, Inc.
  * All rights reserved.
  *
  * For use for simulation and test purposes only
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
  *
  * 1. Redistributions of source code must retain the above copyright notice,
  * this list of conditions and the following disclaimer.
  *
  * 2. Redistributions in binary form must reproduce the above copyright notice,
  * this list of conditions and the following disclaimer in the documentation
  * and/or other materials provided with the distribution.
  *
  * 3. Neither the name of the copyright holder nor the names of its
  * contributors may be used to endorse or promote products derived from this
  * software without specific prior written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
  * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  * POSSIBILITY OF SUCH DAMAGE.
  */

 #ifndef __GPU_TLB_HH__
 #define __GPU_TLB_HH__

 #include <fstream>
 #include <list>
 #include <queue>
 #include <string>
 #include <vector>

 #include "arch/generic/tlb.hh"
 #include "arch/x86/pagetable.hh"
 #include "arch/x86/pagetable_walker.hh"
 #include "arch/x86/regs/segment.hh"
 #include "base/callback.hh"
 #include "base/logging.hh"
 #include "base/statistics.hh"
 #include "base/stats/group.hh"
 #include "gpu-compute/compute_unit.hh"
 #include "mem/port.hh"
 #include "mem/request.hh"
 #include "params/X86GPUTLB.hh"
 #include "sim/clocked_object.hh"
 #include "sim/sim_object.hh"

 namespace gem5
 {

 class BaseTLB;
 class Packet;
 class ThreadContext;

 namespace X86ISA
 {
     class GpuTLB : public ClockedObject
     {
       protected:
         friend class Walker;

         typedef std::list<TlbEntry*> EntryList;

         uint32_t configAddress;

       public:
         typedef X86GPUTLBParams Params;
         GpuTLB(const Params &p);
         ~GpuTLB();

         typedef enum BaseMMU::Mode Mode;

         class Translation
         {
           public:
             virtual ~Translation() { }

             /**
              * Signal that the translation has been delayed due to a hw page
              * table walk.
              */
             virtual void markDelayed() = 0;

             /**
              * The memory for this object may be dynamically allocated, and it
              * may be responsible for cleaning itslef up which will happen in
              * this function. Once it's called the object is no longer valid.
              */
             virtual void finish(Fault fault, const RequestPtr &req,
                                 ThreadContext *tc, Mode mode) = 0;
         };

         void dumpAll();
         TlbEntry *lookup(Addr va, bool update_lru=true);
         void setConfigAddress(uint32_t addr);

       protected:
         EntryList::iterator lookupIt(Addr va, bool update_lru=true);
         Walker *walker;

       public:
         Walker *getWalker();
         void invalidateAll();
         void invalidateNonGlobal();
         void demapPage(Addr va, uint64_t asn);

       protected:
         int size;
         int assoc;
         int numSets;

         /**
          *  true if this is a fully-associative TLB
          */
         bool FA;
         Addr setMask;

         /**
          * Allocation Policy: true if we always allocate on a hit, false
          * otherwise. Default is true.
          */
         bool allocationPolicy;

         /**
          * if true, then this is not the last level TLB
          */
         bool hasMemSidePort;

         /**
          * Print out accessDistance stats. One stat file
          * per TLB.
          */
         bool accessDistance;

         std::vector<TlbEntry> tlb;

         /*
          * It's a per-set list. As long as we have not reached
          * the full capacity of the given set, grab an entry from
          * the freeList.
          */
         std::vector<EntryList> freeList;

         /**
          * An entryList per set is the equivalent of an LRU stack;
          * it's used to guide replacement decisions. The head of the list
          * contains the MRU TLB entry of the given set. If the freeList
          * for this set is empty, the last element of the list
          * is evicted (i.e., dropped on the floor).
          */
         std::vector<EntryList> entryList;

         Fault translateInt(bool read, const RequestPtr &req,
                            ThreadContext *tc);

         Fault translate(const RequestPtr &req, ThreadContext *tc,
                 Translation *translation, Mode mode, bool &delayedResponse,
                 bool timing, int &latency);

       public:
         // latencies for a TLB hit, miss and page fault
         int hitLatency;
         int missLatency1;
         int missLatency2;

         void updatePageFootprint(Addr virt_page_addr);
         void printAccessPattern();


         Fault translateAtomic(const RequestPtr &req, ThreadContext *tc,
                               Mode mode, int &latency);

         void translateTiming(const RequestPtr &req, ThreadContext *tc,
                              Translation *translation, Mode mode,
                              int &latency);

         Tick doMmuRegRead(ThreadContext *tc, Packet *pkt);
         Tick doMmuRegWrite(ThreadContext *tc, Packet *pkt);

         TlbEntry *insert(Addr vpn, TlbEntry &entry);

         // Checkpointing
         virtual void serialize(CheckpointOut& cp) const override;
         virtual void unserialize(CheckpointIn& cp) override;
         void issueTranslation();
         enum tlbOutcome {TLB_HIT, TLB_MISS, PAGE_WALK, MISS_RETURN};
         bool tlbLookup(const RequestPtr &req,
                        ThreadContext *tc, bool update_stats);

         void handleTranslationReturn(Addr addr, tlbOutcome outcome,
                                      PacketPtr pkt);

         void handleFuncTranslationReturn(PacketPtr pkt, tlbOutcome outcome);

         void pagingProtectionChecks(ThreadContext *tc, PacketPtr pkt,
                                     TlbEntry *tlb_entry, Mode mode);

         void updatePhysAddresses(Addr virt_page_addr, TlbEntry *tlb_entry,
                                  Addr phys_page_addr);

         void issueTLBLookup(PacketPtr pkt);

         // CpuSidePort is the TLB Port closer to the CPU/CU side
         class CpuSidePort : public ResponsePort
         {
           public:
             CpuSidePort(const std::string &_name, GpuTLB * gpu_TLB,
                         PortID _index)
                 : ResponsePort(_name, gpu_TLB), tlb(gpu_TLB), index(_index) { }

           protected:
             GpuTLB *tlb;
             int index;

             virtual bool recvTimingReq(PacketPtr pkt);
             virtual Tick recvAtomic(PacketPtr pkt) { return 0; }
             virtual void recvFunctional(PacketPtr pkt);
             virtual void recvRangeChange() { }
             virtual void recvReqRetry();
             virtual void recvRespRetry() { panic("recvRespRetry called"); }
             virtual AddrRangeList getAddrRanges() const;
         };

         /**
          * MemSidePort is the TLB Port closer to the memory side
          * If this is a last level TLB then this port will not be connected.
          *
          * Future action item: if we ever do real page walks, then this port
          * should be connected to a RubyPort.
          */
         class MemSidePort : public RequestPort
         {
           public:
             MemSidePort(const std::string &_name, GpuTLB * gpu_TLB,
                         PortID _index)
                 : RequestPort(_name, gpu_TLB), tlb(gpu_TLB), index(_index) { }

             std::deque<PacketPtr> retries;

           protected:
             GpuTLB *tlb;
             int index;

             virtual bool recvTimingResp(PacketPtr pkt);
             virtual Tick recvAtomic(PacketPtr pkt) { return 0; }
             virtual void recvFunctional(PacketPtr pkt) { }
             virtual void recvRangeChange() { }
             virtual void recvReqRetry();
         };

         // TLB ports on the cpu Side
         std::vector<CpuSidePort*> cpuSidePort;
         // TLB ports on the memory side
         std::vector<MemSidePort*> memSidePort;

         Port &getPort(const std::string &if_name,
                       PortID idx=InvalidPortID) override;

         /**
          * TLB TranslationState: this currently is a somewhat bastardization of
          * the usage of SenderState, whereby the receiver of a packet is not
          * usually supposed to need to look at the contents of the senderState,
          * you're really only supposed to look at what you pushed on, pop it
          * off, and send it back.
          *
          * However, since there is state that we want to pass to the TLBs using
          * the send/recv Timing/Functional/etc. APIs, which don't allow for new
          * arguments, we need a common TLB senderState to pass between TLBs,
          * both "forwards" and "backwards."
          *
          * So, basically, the rule is that any packet received by a TLB port
          * (cpuside OR memside) must be safely castable to a TranslationState.
          */

         struct TranslationState : public Packet::SenderState
         {
             // TLB mode, read or write
             Mode tlbMode;
             // Thread context associated with this req
             ThreadContext *tc;

             /*
             * TLB entry to be populated and passed back and filled in
             * previous TLBs.  Equivalent to the data cache concept of
             * "data return."
             */
             TlbEntry *tlbEntry;
             // Is this a TLB prefetch request?
             bool isPrefetch;
             // When was the req for this translation issued
             uint64_t issueTime;
             // Remember where this came from
             std::vector<ResponsePort*>ports;

             // keep track of #uncoalesced reqs per packet per TLB level;
             // reqCnt per level >= reqCnt higher level
             std::vector<int> reqCnt;
             // TLB level this packet hit in; 0 if it hit in the page table
             int hitLevel;
             Packet::SenderState *saved;

             TranslationState(Mode tlb_mode, ThreadContext *_tc,
                              bool is_prefetch=false,
                              Packet::SenderState *_saved=nullptr)
                 : tlbMode(tlb_mode), tc(_tc), tlbEntry(nullptr),
                   isPrefetch(is_prefetch), issueTime(0),
                   hitLevel(0),saved(_saved) { }
         };

         // maximum number of permitted coalesced requests per cycle
         int maxCoalescedReqs;

         // Current number of outstandings coalesced requests.
         // Should be <= maxCoalescedReqs
         int outstandingReqs;

         /**
          * A TLBEvent is scheduled after the TLB lookup and helps us take the
          * appropriate actions:
          *  (e.g., update TLB on a hit,
          *  send request to lower level TLB on a miss,
          *  or start a page walk if this was the last-level TLB).
          */
         void translationReturn(Addr virtPageAddr, tlbOutcome outcome,
                                PacketPtr pkt);

         class TLBEvent : public Event
         {
             private:
                 GpuTLB *tlb;
                 Addr virtPageAddr;
                 /**
                  * outcome can be TLB_HIT, TLB_MISS, or PAGE_WALK
                  */
                 tlbOutcome outcome;
                 PacketPtr pkt;

             public:
                 TLBEvent(GpuTLB *_tlb, Addr _addr, tlbOutcome outcome,
                         PacketPtr _pkt);

                 void process();
                 const char *description() const;

                 // updateOutcome updates the tlbOutcome of a TLBEvent
                 void updateOutcome(tlbOutcome _outcome);
                 Addr getTLBEventVaddr();
         };

         std::unordered_map<Addr, TLBEvent*> translationReturnEvent;

         // this FIFO queue keeps track of the virt. page addresses
         // that are pending cleanup
         std::queue<Addr> cleanupQueue;

         // the cleanupEvent is scheduled after a TLBEvent triggers in order to
         // free memory and do the required clean-up
         void cleanup();

         EventFunctionWrapper cleanupEvent;

         /**
          * This hash map will use the virtual page address as a key
          * and will keep track of total number of accesses per page
          */

         struct AccessInfo
         {
             unsigned int lastTimeAccessed; // last access to this page
             unsigned int accessesPerPage;
             // need to divide it by accessesPerPage at the end
             unsigned int totalReuseDistance;

             /**
              * The field below will help us compute the access distance,
              * that is the number of (coalesced) TLB accesses that
              * happened in between each access to this page
              *
              * localTLBAccesses[x] is the value of localTLBNumAccesses
              * when the page <Addr> was accessed for the <x>th time
              */
             std::vector<unsigned int> localTLBAccesses;
             unsigned int sumDistance;
             unsigned int meanDistance;
         };

         typedef std::unordered_map<Addr, AccessInfo> AccessPatternTable;
         AccessPatternTable TLBFootprint;

         // Called at the end of simulation to dump page access stats.
         void exitCallback();

         EventFunctionWrapper exitEvent;

       protected:
         struct GpuTLBStats : public statistics::Group
         {
             GpuTLBStats(statistics::Group *parent);

             // local_stats are as seen from the TLB
             // without taking into account coalescing
             statistics::Scalar localNumTLBAccesses;
             statistics::Scalar localNumTLBHits;
             statistics::Scalar localNumTLBMisses;
             statistics::Formula localTLBMissRate;

             // global_stats are as seen from the
             // CU's perspective taking into account
             // all coalesced requests.
             statistics::Scalar globalNumTLBAccesses;
             statistics::Scalar globalNumTLBHits;
             statistics::Scalar globalNumTLBMisses;
             statistics::Formula globalTLBMissRate;

             // from the CU perspective (global)
             statistics::Scalar accessCycles;
             // from the CU perspective (global)
             statistics::Scalar pageTableCycles;
             statistics::Scalar numUniquePages;
             // from the perspective of this TLB
             statistics::Scalar localCycles;
             // from the perspective of this TLB
             statistics::Formula localLatency;
             // I take the avg. per page and then
             // the avg. over all pages.
             statistics::Scalar avgReuseDistance;
         } stats;
     };
 }

 } // namespace gem5

 #endif // __GPU_TLB_HH__
	/*
	* Copyright (c) 2011-2015 Advanced Micro Devices, Inc.
	* All rights reserved.
	*
	* For use for simulation and test purposes only
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions are met:
	*
	* 1. Redistributions of source code must retain the above copyright notice,
	* this list of conditions and the following disclaimer.
	*
	* 2. Redistributions in binary form must reproduce the above copyright notice,
	* this list of conditions and the following disclaimer in the documentation
	* and/or other materials provided with the distribution.
	*
	* 3. Neither the name of the copyright holder nor the names of its
	* contributors may be used to endorse or promote products derived from this
	* software without specific prior written permission.
	*
	* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
	* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
	* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
	* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
	* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
	* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
	* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
	* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
	* POSSIBILITY OF SUCH DAMAGE.
	*/

	#ifndef __GPU_TLB_HH__
	#define __GPU_TLB_HH__

	#include <fstream>
	#include <list>
	#include <queue>
	#include <string>
	#include <vector>

	#include "arch/generic/tlb.hh"
	#include "arch/x86/pagetable.hh"
	#include "arch/x86/pagetable_walker.hh"
	#include "arch/x86/regs/segment.hh"
	#include "base/callback.hh"
	#include "base/logging.hh"
	#include "base/statistics.hh"
	#include "base/stats/group.hh"
	#include "gpu-compute/compute_unit.hh"
	#include "mem/port.hh"
	#include "mem/request.hh"
	#include "params/X86GPUTLB.hh"
	#include "sim/clocked_object.hh"
	#include "sim/sim_object.hh"

	namespace gem5
	{

	class BaseTLB;
	class Packet;
	class ThreadContext;

	namespace X86ISA
	{
	class GpuTLB : public ClockedObject
	{
	protected:
	friend class Walker;

	typedef std::list<TlbEntry*> EntryList;

	uint32_t configAddress;

	public:
	typedef X86GPUTLBParams Params;
	GpuTLB(const Params &p);
	~GpuTLB();

	typedef enum BaseMMU::Mode Mode;

	class Translation
	{
	public:
	virtual ~Translation() { }

	/**
	* Signal that the translation has been delayed due to a hw page
	* table walk.
	*/
	virtual void markDelayed() = 0;

	/**
	* The memory for this object may be dynamically allocated, and it
	* may be responsible for cleaning itslef up which will happen in
	* this function. Once it's called the object is no longer valid.
	*/
	virtual void finish(Fault fault, const RequestPtr &req,
	ThreadContext *tc, Mode mode) = 0;
	};

	void dumpAll();
	TlbEntry *lookup(Addr va, bool update_lru=true);
	void setConfigAddress(uint32_t addr);

	protected:
	EntryList::iterator lookupIt(Addr va, bool update_lru=true);
	Walker *walker;

	public:
	Walker *getWalker();
	void invalidateAll();
	void invalidateNonGlobal();
	void demapPage(Addr va, uint64_t asn);

	protected:
	int size;
	int assoc;
	int numSets;

	/**
	* true if this is a fully-associative TLB
	*/
	bool FA;
	Addr setMask;

	/**
	* Allocation Policy: true if we always allocate on a hit, false
	* otherwise. Default is true.
	*/
	bool allocationPolicy;

	/**
	* if true, then this is not the last level TLB
	*/
	bool hasMemSidePort;

	/**
	* Print out accessDistance stats. One stat file
	* per TLB.
	*/
	bool accessDistance;

	std::vector<TlbEntry> tlb;

	/*
	* It's a per-set list. As long as we have not reached
	* the full capacity of the given set, grab an entry from
	* the freeList.
	*/
	std::vector<EntryList> freeList;

	/**
	* An entryList per set is the equivalent of an LRU stack;
	* it's used to guide replacement decisions. The head of the list
	* contains the MRU TLB entry of the given set. If the freeList
	* for this set is empty, the last element of the list
	* is evicted (i.e., dropped on the floor).
	*/
	std::vector<EntryList> entryList;

	Fault translateInt(bool read, const RequestPtr &req,
	ThreadContext *tc);

	Fault translate(const RequestPtr &req, ThreadContext *tc,
	Translation *translation, Mode mode, bool &delayedResponse,
	bool timing, int &latency);

	public:
	// latencies for a TLB hit, miss and page fault
	int hitLatency;
	int missLatency1;
	int missLatency2;

	void updatePageFootprint(Addr virt_page_addr);
	void printAccessPattern();


	Fault translateAtomic(const RequestPtr &req, ThreadContext *tc,
	Mode mode, int &latency);

	void translateTiming(const RequestPtr &req, ThreadContext *tc,
	Translation *translation, Mode mode,
	int &latency);

	Tick doMmuRegRead(ThreadContext tc, Packet pkt);
	Tick doMmuRegWrite(ThreadContext tc, Packet pkt);

	TlbEntry *insert(Addr vpn, TlbEntry &entry);

	// Checkpointing
	virtual void serialize(CheckpointOut& cp) const override;
	virtual void unserialize(CheckpointIn& cp) override;
	void issueTranslation();
	enum tlbOutcome {TLB_HIT, TLB_MISS, PAGE_WALK, MISS_RETURN};
	bool tlbLookup(const RequestPtr &req,
	ThreadContext *tc, bool update_stats);

	void handleTranslationReturn(Addr addr, tlbOutcome outcome,
	PacketPtr pkt);

	void handleFuncTranslationReturn(PacketPtr pkt, tlbOutcome outcome);

	void pagingProtectionChecks(ThreadContext *tc, PacketPtr pkt,
	TlbEntry *tlb_entry, Mode mode);

	void updatePhysAddresses(Addr virt_page_addr, TlbEntry *tlb_entry,
	Addr phys_page_addr);

	void issueTLBLookup(PacketPtr pkt);

	// CpuSidePort is the TLB Port closer to the CPU/CU side
	class CpuSidePort : public ResponsePort
	{
	public:
	CpuSidePort(const std::string &_name, GpuTLB * gpu_TLB,
	PortID _index)
	: ResponsePort(_name, gpu_TLB), tlb(gpu_TLB), index(_index) { }

	protected:
	GpuTLB *tlb;
	int index;

	virtual bool recvTimingReq(PacketPtr pkt);
	virtual Tick recvAtomic(PacketPtr pkt) { return 0; }
	virtual void recvFunctional(PacketPtr pkt);
	virtual void recvRangeChange() { }
	virtual void recvReqRetry();
	virtual void recvRespRetry() { panic("recvRespRetry called"); }
	virtual AddrRangeList getAddrRanges() const;
	};

	/**
	* MemSidePort is the TLB Port closer to the memory side
	* If this is a last level TLB then this port will not be connected.
	*
	* Future action item: if we ever do real page walks, then this port
	* should be connected to a RubyPort.
	*/
	class MemSidePort : public RequestPort
	{
	public:
	MemSidePort(const std::string &_name, GpuTLB * gpu_TLB,
	PortID _index)
	: RequestPort(_name, gpu_TLB), tlb(gpu_TLB), index(_index) { }

	std::deque<PacketPtr> retries;

	protected:
	GpuTLB *tlb;
	int index;

	virtual bool recvTimingResp(PacketPtr pkt);
	virtual Tick recvAtomic(PacketPtr pkt) { return 0; }
	virtual void recvFunctional(PacketPtr pkt) { }
	virtual void recvRangeChange() { }
	virtual void recvReqRetry();
	};

	// TLB ports on the cpu Side
	std::vector<CpuSidePort*> cpuSidePort;
	// TLB ports on the memory side
	std::vector<MemSidePort*> memSidePort;

	Port &getPort(const std::string &if_name,
	PortID idx=InvalidPortID) override;

	/**
	* TLB TranslationState: this currently is a somewhat bastardization of
	* the usage of SenderState, whereby the receiver of a packet is not
	* usually supposed to need to look at the contents of the senderState,
	* you're really only supposed to look at what you pushed on, pop it
	* off, and send it back.
	*
	* However, since there is state that we want to pass to the TLBs using
	* the send/recv Timing/Functional/etc. APIs, which don't allow for new
	* arguments, we need a common TLB senderState to pass between TLBs,
	* both "forwards" and "backwards."
	*
	* So, basically, the rule is that any packet received by a TLB port
	* (cpuside OR memside) must be safely castable to a TranslationState.
	*/

	struct TranslationState : public Packet::SenderState
	{
	// TLB mode, read or write
	Mode tlbMode;
	// Thread context associated with this req
	ThreadContext *tc;

	/*
	* TLB entry to be populated and passed back and filled in
	* previous TLBs. Equivalent to the data cache concept of
	* "data return."
	*/
	TlbEntry *tlbEntry;
	// Is this a TLB prefetch request?
	bool isPrefetch;
	// When was the req for this translation issued
	uint64_t issueTime;
	// Remember where this came from
	std::vector<ResponsePort*>ports;

	// keep track of #uncoalesced reqs per packet per TLB level;
	// reqCnt per level >= reqCnt higher level
	std::vector<int> reqCnt;
	// TLB level this packet hit in; 0 if it hit in the page table
	int hitLevel;
	Packet::SenderState *saved;

	TranslationState(Mode tlb_mode, ThreadContext *_tc,
	bool is_prefetch=false,
	Packet::SenderState *_saved=nullptr)
	: tlbMode(tlb_mode), tc(_tc), tlbEntry(nullptr),
	isPrefetch(is_prefetch), issueTime(0),
	hitLevel(0),saved(_saved) { }
	};

	// maximum number of permitted coalesced requests per cycle
	int maxCoalescedReqs;

	// Current number of outstandings coalesced requests.
	// Should be <= maxCoalescedReqs
	int outstandingReqs;

	/**
	* A TLBEvent is scheduled after the TLB lookup and helps us take the
	* appropriate actions:
	* (e.g., update TLB on a hit,
	* send request to lower level TLB on a miss,
	* or start a page walk if this was the last-level TLB).
	*/
	void translationReturn(Addr virtPageAddr, tlbOutcome outcome,
	PacketPtr pkt);

	class TLBEvent : public Event
	{
	private:
	GpuTLB *tlb;
	Addr virtPageAddr;
	/**
	* outcome can be TLB_HIT, TLB_MISS, or PAGE_WALK
	*/
	tlbOutcome outcome;
	PacketPtr pkt;

	public:
	TLBEvent(GpuTLB *_tlb, Addr _addr, tlbOutcome outcome,
	PacketPtr _pkt);

	void process();
	const char *description() const;

	// updateOutcome updates the tlbOutcome of a TLBEvent
	void updateOutcome(tlbOutcome _outcome);
	Addr getTLBEventVaddr();
	};

	std::unordered_map<Addr, TLBEvent*> translationReturnEvent;

	// this FIFO queue keeps track of the virt. page addresses
	// that are pending cleanup
	std::queue<Addr> cleanupQueue;

	// the cleanupEvent is scheduled after a TLBEvent triggers in order to
	// free memory and do the required clean-up
	void cleanup();

	EventFunctionWrapper cleanupEvent;

	/**
	* This hash map will use the virtual page address as a key
	* and will keep track of total number of accesses per page
	*/

	struct AccessInfo
	{
	unsigned int lastTimeAccessed; // last access to this page
	unsigned int accessesPerPage;
	// need to divide it by accessesPerPage at the end
	unsigned int totalReuseDistance;

	/**
	* The field below will help us compute the access distance,
	* that is the number of (coalesced) TLB accesses that
	* happened in between each access to this page
	*
	* localTLBAccesses[x] is the value of localTLBNumAccesses
	* when the page <Addr> was accessed for the <x>th time
	*/
	std::vector<unsigned int> localTLBAccesses;
	unsigned int sumDistance;
	unsigned int meanDistance;
	};

	typedef std::unordered_map<Addr, AccessInfo> AccessPatternTable;
	AccessPatternTable TLBFootprint;

	// Called at the end of simulation to dump page access stats.
	void exitCallback();

	EventFunctionWrapper exitEvent;

	protected:
	struct GpuTLBStats : public statistics::Group
	{
	GpuTLBStats(statistics::Group *parent);

	// local_stats are as seen from the TLB
	// without taking into account coalescing
	statistics::Scalar localNumTLBAccesses;
	statistics::Scalar localNumTLBHits;
	statistics::Scalar localNumTLBMisses;
	statistics::Formula localTLBMissRate;

	// global_stats are as seen from the
	// CU's perspective taking into account
	// all coalesced requests.
	statistics::Scalar globalNumTLBAccesses;
	statistics::Scalar globalNumTLBHits;
	statistics::Scalar globalNumTLBMisses;
	statistics::Formula globalTLBMissRate;

	// from the CU perspective (global)
	statistics::Scalar accessCycles;
	// from the CU perspective (global)
	statistics::Scalar pageTableCycles;
	statistics::Scalar numUniquePages;
	// from the perspective of this TLB
	statistics::Scalar localCycles;
	// from the perspective of this TLB
	statistics::Formula localLatency;
	// I take the avg. per page and then
	// the avg. over all pages.
	statistics::Scalar avgReuseDistance;
	} stats;
	};
	}

	} // namespace gem5

	#endif // __GPU_TLB_HH__