src/mem/coherent_xbar.hh - public/gem5 - Git at Google

 /*
  * Copyright (c) 2011-2015, 2017, 2019 ARM Limited
  * All rights reserved
  *
  * The license below extends only to copyright in the software and shall
  * not be construed as granting a license to any other intellectual
  * property including but not limited to intellectual property relating
  * to a hardware implementation of the functionality of the software
  * licensed hereunder.  You may use the software subject to the license
  * terms below provided that you ensure that this notice is replicated
  * unmodified and in its entirety in all distributions of the software,
  * modified or unmodified, in source code or in binary form.
  *
  * Copyright (c) 2002-2005 The Regents of The University of Michigan
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met: redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer;
  * 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;
  * neither the name of the copyright holders 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
  * OWNER 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.
  */

 /**
  * @file
  * Declaration of a coherent crossbar.
  */

 #ifndef __MEM_COHERENT_XBAR_HH__
 #define __MEM_COHERENT_XBAR_HH__

 #include <unordered_map>
 #include <unordered_set>

 #include "mem/snoop_filter.hh"
 #include "mem/xbar.hh"
 #include "params/CoherentXBar.hh"

 /**
  * A coherent crossbar connects a number of (potentially) snooping
  * masters and slaves, and routes the request and response packets
  * based on the address, and also forwards all requests to the
  * snoopers and deals with the snoop responses.
  *
  * The coherent crossbar can be used as a template for modelling QPI,
  * HyperTransport, ACE and coherent OCP buses, and is typically used
  * for the L1-to-L2 buses and as the main system interconnect.  @sa
  * \ref gem5MemorySystem "gem5 Memory System"
  */
 class CoherentXBar : public BaseXBar
 {

   protected:

     /**
      * Declare the layers of this crossbar, one vector for requests,
      * one for responses, and one for snoop responses
      */
     std::vector<ReqLayer*> reqLayers;
     std::vector<RespLayer*> respLayers;
     std::vector<SnoopRespLayer*> snoopLayers;

     /**
      * Declaration of the coherent crossbar slave port type, one will
      * be instantiated for each of the master ports connecting to the
      * crossbar.
      */
     class CoherentXBarSlavePort : public QueuedSlavePort
     {

       private:

         /** A reference to the crossbar to which this port belongs. */
         CoherentXBar &xbar;

         /** A normal packet queue used to store responses. */
         RespPacketQueue queue;

       public:

         CoherentXBarSlavePort(const std::string &_name,
                              CoherentXBar &_xbar, PortID _id)
             : QueuedSlavePort(_name, &_xbar, queue, _id), xbar(_xbar),
               queue(_xbar, *this)
         { }

       protected:

         bool
         recvTimingReq(PacketPtr pkt) override
         {
             return xbar.recvTimingReq(pkt, id);
         }

         bool
         recvTimingSnoopResp(PacketPtr pkt) override
         {
             return xbar.recvTimingSnoopResp(pkt, id);
         }

         Tick
         recvAtomic(PacketPtr pkt) override
         {
             return xbar.recvAtomicBackdoor(pkt, id);
         }

         Tick
         recvAtomicBackdoor(PacketPtr pkt, MemBackdoorPtr &backdoor) override
         {
             return xbar.recvAtomicBackdoor(pkt, id, &backdoor);
         }

         void
         recvFunctional(PacketPtr pkt) override
         {
             xbar.recvFunctional(pkt, id);
         }

         AddrRangeList
         getAddrRanges() const override
         {
             return xbar.getAddrRanges();
         }

     };

     /**
      * Declaration of the coherent crossbar master port type, one will be
      * instantiated for each of the slave interfaces connecting to the
      * crossbar.
      */
     class CoherentXBarMasterPort : public RequestPort
     {
       private:
         /** A reference to the crossbar to which this port belongs. */
         CoherentXBar &xbar;

       public:

         CoherentXBarMasterPort(const std::string &_name,
                               CoherentXBar &_xbar, PortID _id)
             : RequestPort(_name, &_xbar, _id), xbar(_xbar)
         { }

       protected:

         /**
          * Determine if this port should be considered a snooper. For
          * a coherent crossbar master port this is always true.
          *
          * @return a boolean that is true if this port is snooping
          */
         bool isSnooping() const override { return true; }

         bool
         recvTimingResp(PacketPtr pkt) override
         {
             return xbar.recvTimingResp(pkt, id);
         }

         void
         recvTimingSnoopReq(PacketPtr pkt) override
         {
             return xbar.recvTimingSnoopReq(pkt, id);
         }

         Tick
         recvAtomicSnoop(PacketPtr pkt) override
         {
             return xbar.recvAtomicSnoop(pkt, id);
         }

         void
         recvFunctionalSnoop(PacketPtr pkt) override
         {
             xbar.recvFunctionalSnoop(pkt, id);
         }

         void recvRangeChange() override { xbar.recvRangeChange(id); }
         void recvReqRetry() override { xbar.recvReqRetry(id); }

     };

     /**
      * Internal class to bridge between an incoming snoop response
      * from a slave port and forwarding it through an outgoing slave
      * port. It is effectively a dangling master port.
      */
     class SnoopRespPort : public RequestPort
     {

       private:

         /** The port which we mirror internally. */
         QueuedSlavePort& slavePort;

       public:

         /**
          * Create a snoop response port that mirrors a given slave port.
          */
         SnoopRespPort(QueuedSlavePort& slave_port, CoherentXBar& _xbar) :
             RequestPort(slave_port.name() + ".snoopRespPort", &_xbar),
             slavePort(slave_port) { }

         /**
          * Override the sending of retries and pass them on through
          * the mirrored slave port.
          */
         void
         sendRetryResp() override
         {
             // forward it as a snoop response retry
             slavePort.sendRetrySnoopResp();
         }

         void
         recvReqRetry() override
         {
             panic("SnoopRespPort should never see retry");
         }

         bool
         recvTimingResp(PacketPtr pkt) override
         {
             panic("SnoopRespPort should never see timing response");
         }

     };

     std::vector<SnoopRespPort*> snoopRespPorts;

     std::vector<QueuedSlavePort*> snoopPorts;

     /**
      * Store the outstanding requests that we are expecting snoop
      * responses from so we can determine which snoop responses we
      * generated and which ones were merely forwarded.
      */
     std::unordered_set<RequestPtr> outstandingSnoop;

     /**
      * Store the outstanding cache maintenance that we are expecting
      * snoop responses from so we can determine when we received all
      * snoop responses and if any of the agents satisfied the request.
      */
     std::unordered_map<PacketId, PacketPtr> outstandingCMO;

     /**
      * Keep a pointer to the system to be allow to querying memory system
      * properties.
      */
     System *system;

     /** A snoop filter that tracks cache line residency and can restrict the
       * broadcast needed for probes.  NULL denotes an absent filter. */
     SnoopFilter *snoopFilter;

     /** Cycles of snoop response latency.*/
     const Cycles snoopResponseLatency;

     /** Maximum number of outstading snoops sanity check*/
     const unsigned int maxOutstandingSnoopCheck;

     /** Maximum routing table size sanity check*/
     const unsigned int maxRoutingTableSizeCheck;

     /** Is this crossbar the point of coherency? **/
     const bool pointOfCoherency;

     /** Is this crossbar the point of unification? **/
     const bool pointOfUnification;

     /**
      * Upstream caches need this packet until true is returned, so
      * hold it for deletion until a subsequent call
      */
     std::unique_ptr<Packet> pendingDelete;

     bool recvTimingReq(PacketPtr pkt, PortID slave_port_id);
     bool recvTimingResp(PacketPtr pkt, PortID master_port_id);
     void recvTimingSnoopReq(PacketPtr pkt, PortID master_port_id);
     bool recvTimingSnoopResp(PacketPtr pkt, PortID slave_port_id);
     void recvReqRetry(PortID master_port_id);

     /**
      * Forward a timing packet to our snoopers, potentially excluding
      * one of the connected coherent masters to avoid sending a packet
      * back to where it came from.
      *
      * @param pkt Packet to forward
      * @param exclude_slave_port_id Id of slave port to exclude
      */
     void
     forwardTiming(PacketPtr pkt, PortID exclude_slave_port_id)
     {
         forwardTiming(pkt, exclude_slave_port_id, snoopPorts);
     }

     /**
      * Forward a timing packet to a selected list of snoopers, potentially
      * excluding one of the connected coherent masters to avoid sending a packet
      * back to where it came from.
      *
      * @param pkt Packet to forward
      * @param exclude_slave_port_id Id of slave port to exclude
      * @param dests Vector of destination ports for the forwarded pkt
      */
     void forwardTiming(PacketPtr pkt, PortID exclude_slave_port_id,
                        const std::vector<QueuedSlavePort*>& dests);

     Tick recvAtomicBackdoor(PacketPtr pkt, PortID slave_port_id,
                             MemBackdoorPtr *backdoor=nullptr);
     Tick recvAtomicSnoop(PacketPtr pkt, PortID master_port_id);

     /**
      * Forward an atomic packet to our snoopers, potentially excluding
      * one of the connected coherent masters to avoid sending a packet
      * back to where it came from.
      *
      * @param pkt Packet to forward
      * @param exclude_slave_port_id Id of slave port to exclude
      *
      * @return a pair containing the snoop response and snoop latency
      */
     std::pair<MemCmd, Tick>
     forwardAtomic(PacketPtr pkt, PortID exclude_slave_port_id)
     {
         return forwardAtomic(pkt, exclude_slave_port_id, InvalidPortID,
                              snoopPorts);
     }

     /**
      * Forward an atomic packet to a selected list of snoopers, potentially
      * excluding one of the connected coherent masters to avoid sending a packet
      * back to where it came from.
      *
      * @param pkt Packet to forward
      * @param exclude_slave_port_id Id of slave port to exclude
      * @param source_master_port_id Id of the master port for snoops from below
      * @param dests Vector of destination ports for the forwarded pkt
      *
      * @return a pair containing the snoop response and snoop latency
      */
     std::pair<MemCmd, Tick> forwardAtomic(PacketPtr pkt,
                                           PortID exclude_slave_port_id,
                                           PortID source_master_port_id,
                                           const std::vector<QueuedSlavePort*>&
                                           dests);

     /** Function called by the port when the crossbar is recieving a Functional
         transaction.*/
     void recvFunctional(PacketPtr pkt, PortID slave_port_id);

     /** Function called by the port when the crossbar is recieving a functional
         snoop transaction.*/
     void recvFunctionalSnoop(PacketPtr pkt, PortID master_port_id);

     /**
      * Forward a functional packet to our snoopers, potentially
      * excluding one of the connected coherent masters to avoid
      * sending a packet back to where it came from.
      *
      * @param pkt Packet to forward
      * @param exclude_slave_port_id Id of slave port to exclude
      */
     void forwardFunctional(PacketPtr pkt, PortID exclude_slave_port_id);

     /**
      * Determine if the crossbar should sink the packet, as opposed to
      * forwarding it, or responding.
      */
     bool sinkPacket(const PacketPtr pkt) const;

     /**
      * Determine if the crossbar should forward the packet, as opposed to
      * responding to it.
      */
     bool forwardPacket(const PacketPtr pkt);

     /**
      * Determine if the packet's destination is the memory below
      *
      * The memory below is the destination for a cache mainteance
      * operation to the Point of Coherence/Unification if this is the
      * Point of Coherence/Unification.
      *
      * @param pkt The processed packet
      *
      * @return Whether the memory below is the destination for the packet
      */
     bool
     isDestination(const PacketPtr pkt) const
     {
         return (pkt->req->isToPOC() && pointOfCoherency) ||
             (pkt->req->isToPOU() && pointOfUnification);
     }

     Stats::Scalar snoops;
     Stats::Scalar snoopTraffic;
     Stats::Distribution snoopFanout;

   public:

     virtual void init();

     CoherentXBar(const CoherentXBarParams *p);

     virtual ~CoherentXBar();

     virtual void regStats();
 };

 #endif //__MEM_COHERENT_XBAR_HH__
	/*
	* Copyright (c) 2011-2015, 2017, 2019 ARM Limited
	* All rights reserved
	*
	* The license below extends only to copyright in the software and shall
	* not be construed as granting a license to any other intellectual
	* property including but not limited to intellectual property relating
	* to a hardware implementation of the functionality of the software
	* licensed hereunder. You may use the software subject to the license
	* terms below provided that you ensure that this notice is replicated
	* unmodified and in its entirety in all distributions of the software,
	* modified or unmodified, in source code or in binary form.
	*
	* Copyright (c) 2002-2005 The Regents of The University of Michigan
	* All rights reserved.
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions are
	* met: redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer;
	* 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;
	* neither the name of the copyright holders 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
	* OWNER 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.
	*/

	/**
	* @file
	* Declaration of a coherent crossbar.
	*/

	#ifndef __MEM_COHERENT_XBAR_HH__
	#define __MEM_COHERENT_XBAR_HH__

	#include <unordered_map>
	#include <unordered_set>

	#include "mem/snoop_filter.hh"
	#include "mem/xbar.hh"
	#include "params/CoherentXBar.hh"

	/**
	* A coherent crossbar connects a number of (potentially) snooping
	* masters and slaves, and routes the request and response packets
	* based on the address, and also forwards all requests to the
	* snoopers and deals with the snoop responses.
	*
	* The coherent crossbar can be used as a template for modelling QPI,
	* HyperTransport, ACE and coherent OCP buses, and is typically used
	* for the L1-to-L2 buses and as the main system interconnect. @sa
	* \ref gem5MemorySystem "gem5 Memory System"
	*/
	class CoherentXBar : public BaseXBar
	{

	protected:

	/**
	* Declare the layers of this crossbar, one vector for requests,
	* one for responses, and one for snoop responses
	*/
	std::vector<ReqLayer*> reqLayers;
	std::vector<RespLayer*> respLayers;
	std::vector<SnoopRespLayer*> snoopLayers;

	/**
	* Declaration of the coherent crossbar slave port type, one will
	* be instantiated for each of the master ports connecting to the
	* crossbar.
	*/
	class CoherentXBarSlavePort : public QueuedSlavePort
	{

	private:

	/** A reference to the crossbar to which this port belongs. */
	CoherentXBar &xbar;

	/** A normal packet queue used to store responses. */
	RespPacketQueue queue;

	public:

	CoherentXBarSlavePort(const std::string &_name,
	CoherentXBar &_xbar, PortID _id)
	: QueuedSlavePort(_name, &_xbar, queue, _id), xbar(_xbar),
	queue(_xbar, *this)
	{ }

	protected:

	bool
	recvTimingReq(PacketPtr pkt) override
	{
	return xbar.recvTimingReq(pkt, id);
	}

	bool
	recvTimingSnoopResp(PacketPtr pkt) override
	{
	return xbar.recvTimingSnoopResp(pkt, id);
	}

	Tick
	recvAtomic(PacketPtr pkt) override
	{
	return xbar.recvAtomicBackdoor(pkt, id);
	}

	Tick
	recvAtomicBackdoor(PacketPtr pkt, MemBackdoorPtr &backdoor) override
	{
	return xbar.recvAtomicBackdoor(pkt, id, &backdoor);
	}

	void
	recvFunctional(PacketPtr pkt) override
	{
	xbar.recvFunctional(pkt, id);
	}

	AddrRangeList
	getAddrRanges() const override
	{
	return xbar.getAddrRanges();
	}

	};

	/**
	* Declaration of the coherent crossbar master port type, one will be
	* instantiated for each of the slave interfaces connecting to the
	* crossbar.
	*/
	class CoherentXBarMasterPort : public RequestPort
	{
	private:
	/** A reference to the crossbar to which this port belongs. */
	CoherentXBar &xbar;

	public:

	CoherentXBarMasterPort(const std::string &_name,
	CoherentXBar &_xbar, PortID _id)
	: RequestPort(_name, &_xbar, _id), xbar(_xbar)
	{ }

	protected:

	/**
	* Determine if this port should be considered a snooper. For
	* a coherent crossbar master port this is always true.
	*
	* @return a boolean that is true if this port is snooping
	*/
	bool isSnooping() const override { return true; }

	bool
	recvTimingResp(PacketPtr pkt) override
	{
	return xbar.recvTimingResp(pkt, id);
	}

	void
	recvTimingSnoopReq(PacketPtr pkt) override
	{
	return xbar.recvTimingSnoopReq(pkt, id);
	}

	Tick
	recvAtomicSnoop(PacketPtr pkt) override
	{
	return xbar.recvAtomicSnoop(pkt, id);
	}

	void
	recvFunctionalSnoop(PacketPtr pkt) override
	{
	xbar.recvFunctionalSnoop(pkt, id);
	}

	void recvRangeChange() override { xbar.recvRangeChange(id); }
	void recvReqRetry() override { xbar.recvReqRetry(id); }

	};

	/**
	* Internal class to bridge between an incoming snoop response
	* from a slave port and forwarding it through an outgoing slave
	* port. It is effectively a dangling master port.
	*/
	class SnoopRespPort : public RequestPort
	{

	private:

	/** The port which we mirror internally. */
	QueuedSlavePort& slavePort;

	public:

	/**
	* Create a snoop response port that mirrors a given slave port.
	*/
	SnoopRespPort(QueuedSlavePort& slave_port, CoherentXBar& _xbar) :
	RequestPort(slave_port.name() + ".snoopRespPort", &_xbar),
	slavePort(slave_port) { }

	/**
	* Override the sending of retries and pass them on through
	* the mirrored slave port.
	*/
	void
	sendRetryResp() override
	{
	// forward it as a snoop response retry
	slavePort.sendRetrySnoopResp();
	}

	void
	recvReqRetry() override
	{
	panic("SnoopRespPort should never see retry");
	}

	bool
	recvTimingResp(PacketPtr pkt) override
	{
	panic("SnoopRespPort should never see timing response");
	}

	};

	std::vector<SnoopRespPort*> snoopRespPorts;

	std::vector<QueuedSlavePort*> snoopPorts;

	/**
	* Store the outstanding requests that we are expecting snoop
	* responses from so we can determine which snoop responses we
	* generated and which ones were merely forwarded.
	*/
	std::unordered_set<RequestPtr> outstandingSnoop;

	/**
	* Store the outstanding cache maintenance that we are expecting
	* snoop responses from so we can determine when we received all
	* snoop responses and if any of the agents satisfied the request.
	*/
	std::unordered_map<PacketId, PacketPtr> outstandingCMO;

	/**
	* Keep a pointer to the system to be allow to querying memory system
	* properties.
	*/
	System *system;

	/** A snoop filter that tracks cache line residency and can restrict the
	* broadcast needed for probes. NULL denotes an absent filter. */
	SnoopFilter *snoopFilter;

	/** Cycles of snoop response latency.*/
	const Cycles snoopResponseLatency;

	/** Maximum number of outstading snoops sanity check*/
	const unsigned int maxOutstandingSnoopCheck;

	/** Maximum routing table size sanity check*/
	const unsigned int maxRoutingTableSizeCheck;

	/ Is this crossbar the point of coherency? /
	const bool pointOfCoherency;

	/ Is this crossbar the point of unification? /
	const bool pointOfUnification;

	/**
	* Upstream caches need this packet until true is returned, so
	* hold it for deletion until a subsequent call
	*/
	std::unique_ptr<Packet> pendingDelete;

	bool recvTimingReq(PacketPtr pkt, PortID slave_port_id);
	bool recvTimingResp(PacketPtr pkt, PortID master_port_id);
	void recvTimingSnoopReq(PacketPtr pkt, PortID master_port_id);
	bool recvTimingSnoopResp(PacketPtr pkt, PortID slave_port_id);
	void recvReqRetry(PortID master_port_id);

	/**
	* Forward a timing packet to our snoopers, potentially excluding
	* one of the connected coherent masters to avoid sending a packet
	* back to where it came from.
	*
	* @param pkt Packet to forward
	* @param exclude_slave_port_id Id of slave port to exclude
	*/
	void
	forwardTiming(PacketPtr pkt, PortID exclude_slave_port_id)
	{
	forwardTiming(pkt, exclude_slave_port_id, snoopPorts);
	}

	/**
	* Forward a timing packet to a selected list of snoopers, potentially
	* excluding one of the connected coherent masters to avoid sending a packet
	* back to where it came from.
	*
	* @param pkt Packet to forward
	* @param exclude_slave_port_id Id of slave port to exclude
	* @param dests Vector of destination ports for the forwarded pkt
	*/
	void forwardTiming(PacketPtr pkt, PortID exclude_slave_port_id,
	const std::vector<QueuedSlavePort*>& dests);

	Tick recvAtomicBackdoor(PacketPtr pkt, PortID slave_port_id,
	MemBackdoorPtr *backdoor=nullptr);
	Tick recvAtomicSnoop(PacketPtr pkt, PortID master_port_id);

	/**
	* Forward an atomic packet to our snoopers, potentially excluding
	* one of the connected coherent masters to avoid sending a packet
	* back to where it came from.
	*
	* @param pkt Packet to forward
	* @param exclude_slave_port_id Id of slave port to exclude
	*
	* @return a pair containing the snoop response and snoop latency
	*/
	std::pair<MemCmd, Tick>
	forwardAtomic(PacketPtr pkt, PortID exclude_slave_port_id)
	{
	return forwardAtomic(pkt, exclude_slave_port_id, InvalidPortID,
	snoopPorts);
	}

	/**
	* Forward an atomic packet to a selected list of snoopers, potentially
	* excluding one of the connected coherent masters to avoid sending a packet
	* back to where it came from.
	*
	* @param pkt Packet to forward
	* @param exclude_slave_port_id Id of slave port to exclude
	* @param source_master_port_id Id of the master port for snoops from below
	* @param dests Vector of destination ports for the forwarded pkt
	*
	* @return a pair containing the snoop response and snoop latency
	*/
	std::pair<MemCmd, Tick> forwardAtomic(PacketPtr pkt,
	PortID exclude_slave_port_id,
	PortID source_master_port_id,
	const std::vector<QueuedSlavePort*>&
	dests);

	/** Function called by the port when the crossbar is recieving a Functional
	transaction.*/
	void recvFunctional(PacketPtr pkt, PortID slave_port_id);

	/** Function called by the port when the crossbar is recieving a functional
	snoop transaction.*/
	void recvFunctionalSnoop(PacketPtr pkt, PortID master_port_id);

	/**
	* Forward a functional packet to our snoopers, potentially
	* excluding one of the connected coherent masters to avoid
	* sending a packet back to where it came from.
	*
	* @param pkt Packet to forward
	* @param exclude_slave_port_id Id of slave port to exclude
	*/
	void forwardFunctional(PacketPtr pkt, PortID exclude_slave_port_id);

	/**
	* Determine if the crossbar should sink the packet, as opposed to
	* forwarding it, or responding.
	*/
	bool sinkPacket(const PacketPtr pkt) const;

	/**
	* Determine if the crossbar should forward the packet, as opposed to
	* responding to it.
	*/
	bool forwardPacket(const PacketPtr pkt);

	/**
	* Determine if the packet's destination is the memory below
	*
	* The memory below is the destination for a cache mainteance
	* operation to the Point of Coherence/Unification if this is the
	* Point of Coherence/Unification.
	*
	* @param pkt The processed packet
	*
	* @return Whether the memory below is the destination for the packet
	*/
	bool
	isDestination(const PacketPtr pkt) const
	{
	return (pkt->req->isToPOC() && pointOfCoherency) \|\|
	(pkt->req->isToPOU() && pointOfUnification);
	}

	Stats::Scalar snoops;
	Stats::Scalar snoopTraffic;
	Stats::Distribution snoopFanout;

	public:

	virtual void init();

	CoherentXBar(const CoherentXBarParams *p);

	virtual ~CoherentXBar();

	virtual void regStats();
	};

	#endif //__MEM_COHERENT_XBAR_HH__