xref: /gem5/src/mem/snoop_filter.hh

/*
 * Copyright (c) 2013-2015 ARM Limited
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 * this software without specific prior written permission.
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 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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 * Authors: Stephan Diestelhorst
 */

/**
 * @file
 * Definition of a snoop filter.
 */

#ifndef __MEM_SNOOP_FILTER_HH__
#define __MEM_SNOOP_FILTER_HH__

#include <unordered_map>
#include <utility>

#include "mem/packet.hh"
#include "mem/port.hh"
#include "mem/qport.hh"
#include "params/SnoopFilter.hh"
#include "sim/sim_object.hh"
#include "sim/system.hh"

/**
 * This snoop filter keeps track of which connected port has a
 * particular line of data. It can be queried (through lookup*) on
 * memory requests from above (reads / writes / ...); and also from
 * below (snoops). The snoop filter precisely knows about the location
 * of lines "above" it through a map from cache line address to
 * sharers/ports. The snoop filter ties into the flows of requests
 * (when they succeed at the lower interface), regular responses from
 * below and also responses from sideway's caches (in update*). This
 * allows the snoop filter to model cache-line residency by snooping
 * the messages.
 *
 * The tracking happens in two fields to be able to distinguish
 * between in-flight requests (in requested) and already pulled in
 * lines (in holder). This distinction is used for producing tighter
 * assertions and tracking request completion. For safety, (requested
 * | holder) should be notified and the requesting MSHRs will take
 * care of ordering.
 *
 * Overall, some trickery is required because:
 * (1) snoops are not followed by an ACK, but only evoke a response if
 *     they need to (hit dirty)
 * (2) side-channel information is funnelled through direct modifications of
 *     pkt, instead of proper messages through the bus
 * (3) there are no clean evict messages telling the snoop filter that a local,
 *     upper cache dropped a line, making the snoop filter pessimistic for now
 * (4) ordering: there is no single point of order in the system.  Instead,
 *     requesting MSHRs track order between local requests and remote snoops
 */
class SnoopFilter : public SimObject {
  public:
    typedef std::vector<QueuedSlavePort*> SnoopList;

    SnoopFilter (const SnoopFilterParams *p) :
        SimObject(p), reqLookupResult(cachedLocations.end()), retryItem{0, 0},
        linesize(p->system->cacheLineSize()), lookupLatency(p->lookup_latency),
        maxEntryCount(p->max_capacity / p->system->cacheLineSize())
    {
    }

    /**
     * Init a new snoop filter and tell it about all the slave ports
     * of the enclosing bus.
     *
     * @param slave_ports Slave ports that the bus is attached to.
     */
    void setSlavePorts(const SnoopList& slave_ports) {
        localSlavePortIds.resize(slave_ports.size(), InvalidPortID);

        PortID id = 0;
        for (const auto& p : slave_ports) {
            // no need to track this port if it is not snooping
            if (p->isSnooping()) {
                slavePorts.push_back(p);
                localSlavePortIds[p->getId()] = id++;
            }
        }

        // make sure we can deal with this many ports
        fatal_if(id > 8 * sizeof(SnoopMask),
                 "Snoop filter only supports %d snooping ports, got %d\n",
                 8 * sizeof(SnoopMask), id);
    }

    /**
     * Lookup a request (from a slave port) in the snoop filter and
     * return a list of other slave ports that need forwarding of the
     * resulting snoops.  Additionally, update the tracking structures
     * with new request information. Note that the caller must also
     * call finishRequest once it is known if the request needs to
     * retry or not.
     *
     * @param cpkt          Pointer to the request packet. Not changed.
     * @param slave_port    Slave port where the request came from.
     * @return Pair of a vector of snoop target ports and lookup latency.
     */
    std::pair<SnoopList, Cycles> lookupRequest(const Packet* cpkt,
                                               const SlavePort& slave_port);

    /**
     * For an un-successful request, revert the change to the snoop
     * filter. Also take care of erasing any null entries. This method
     * relies on the result from lookupRequest being stored in
     * reqLookupResult.
     *
     * @param will_retry    This request will retry on this bus / snoop filter
     * @param cpkt          Request packet, merely for sanity checking
     */
    void finishRequest(bool will_retry, const Packet* cpkt);

    /**
     * Handle an incoming snoop from below (the master port). These
     * can upgrade the tracking logic and may also benefit from
     * additional steering thanks to the snoop filter.
     *
     * @param cpkt Pointer to const Packet containing the snoop.
     * @return Pair with a vector of SlavePorts that need snooping and a lookup
     *         latency.
     */
    std::pair<SnoopList, Cycles> lookupSnoop(const Packet* cpkt);

    /**
     * Let the snoop filter see any snoop responses that turn into
     * request responses and indicate cache to cache transfers. These
     * will update the corresponding state in the filter.
     *
     * @param cpkt     Pointer to const Packet holding the snoop response.
     * @param rsp_port SlavePort that sends the response.
     * @param req_port SlavePort that made the original request and is the
     *                 destination of the snoop response.
     */
    void updateSnoopResponse(const Packet *cpkt, const SlavePort& rsp_port,
                             const SlavePort& req_port);

    /**
     * Pass snoop responses that travel downward through the snoop
     * filter and let them update the snoop filter state.  No
     * additional routing happens.
     *
     * @param cpkt     Pointer to const Packet holding the snoop response.
     * @param rsp_port SlavePort that sends the response.
     * @param req_port MasterPort through which the response is forwarded.
     */
    void updateSnoopForward(const Packet *cpkt, const SlavePort& rsp_port,
                            const MasterPort& req_port);

    /**
     * Update the snoop filter with a response from below (outer /
     * other cache, or memory) and update the tracking information in
     * the snoop filter.
     *
     * @param cpkt       Pointer to const Packet holding the snoop response.
     * @param slave_port SlavePort that made the original request and
     *                   is the target of this response.
     */
    void updateResponse(const Packet *cpkt, const SlavePort& slave_port);

    virtual void regStats();

  protected:

    /**
     * The underlying type for the bitmask we use for tracking. This
     * limits the number of snooping ports supported per crossbar. For
     * the moment it is an uint64_t to offer maximum
     * scalability. However, it is possible to use e.g. a uint16_t or
     * uint32_to slim down the footprint of the hash map (and
     * ultimately improve the simulation performance).
     */
    typedef uint64_t SnoopMask;

    /**
    * Per cache line item tracking a bitmask of SlavePorts who have an
    * outstanding request to this line (requested) or already share a
    * cache line with this address (holder).
    */
    struct SnoopItem {
        SnoopMask requested;
        SnoopMask holder;
    };
    /**
     * HashMap of SnoopItems indexed by line address
     */
    typedef std::unordered_map<Addr, SnoopItem> SnoopFilterCache;

    /**
     * Simple factory methods for standard return values.
     */
    std::pair<SnoopList, Cycles> snoopAll(Cycles latency) const
    {
        return std::make_pair(slavePorts, latency);
    }
    std::pair<SnoopList, Cycles> snoopSelected(const SnoopList& slave_ports,
                                               Cycles latency) const
    {
        return std::make_pair(slave_ports, latency);
    }
    std::pair<SnoopList, Cycles> snoopDown(Cycles latency) const
    {
        SnoopList empty;
        return std::make_pair(empty , latency);
    }

    /**
     * Convert a single port to a corresponding, one-hot bitmask
     * @param port SlavePort that should be converted.
     * @return One-hot bitmask corresponding to the port.
     */
    SnoopMask portToMask(const SlavePort& port) const;
    /**
     * Converts a bitmask of ports into the corresponing list of ports
     * @param ports SnoopMask of the requested ports
     * @return SnoopList containing all the requested SlavePorts
     */
    SnoopList maskToPortList(SnoopMask ports) const;

  private:

    /**
     * Removes snoop filter items which have no requesters and no holders.
     */
    void eraseIfNullEntry(SnoopFilterCache::iterator& sf_it);

    /** Simple hash set of cached addresses. */
    SnoopFilterCache cachedLocations;
    /**
     * Iterator used to store the result from lookupRequest until we
     * call finishRequest.
     */
    SnoopFilterCache::iterator reqLookupResult;
    /**
     * Variable to temporarily store value of snoopfilter entry
     * incase finishRequest needs to undo changes made in lookupRequest
     * (because of crossbar retry)
     */
    SnoopItem retryItem;
    /** List of all attached snooping slave ports. */
    SnoopList slavePorts;
    /** Track the mapping from port ids to the local mask ids. */
    std::vector<PortID> localSlavePortIds;
    /** Cache line size. */
    const unsigned linesize;
    /** Latency for doing a lookup in the filter */
    const Cycles lookupLatency;
    /** Max capacity in terms of cache blocks tracked, for sanity checking */
    const unsigned maxEntryCount;

    /** Statistics */
    Stats::Scalar totRequests;
    Stats::Scalar hitSingleRequests;
    Stats::Scalar hitMultiRequests;

    Stats::Scalar totSnoops;
    Stats::Scalar hitSingleSnoops;
    Stats::Scalar hitMultiSnoops;
};

inline SnoopFilter::SnoopMask
SnoopFilter::portToMask(const SlavePort& port) const
{
    assert(port.getId() != InvalidPortID);
    // if this is not a snooping port, return a zero mask
    return !port.isSnooping() ? 0 :
        ((SnoopMask)1) << localSlavePortIds[port.getId()];
}

inline SnoopFilter::SnoopList
SnoopFilter::maskToPortList(SnoopMask port_mask) const
{
    SnoopList res;
    for (const auto& p : slavePorts)
        if (port_mask & portToMask(*p))
            res.push_back(p);
    return res;
}

#endif // __MEM_SNOOP_FILTER_HH__