snoop_filter.hh revision 11133:81e46b63daff 
1/*
2 * Copyright (c) 2013-2015 ARM Limited
3 * All rights reserved
4 *
5 * The license below extends only to copyright in the software and shall
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9 * licensed hereunder.  You may use the software subject to the license
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15 * modification, are permitted provided that the following conditions are
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23 * this software without specific prior written permission.
24 *
25 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
26 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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31 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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35 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
36 *
37 * Authors: Stephan Diestelhorst <stephan.diestelhorst@arm.com>
38 */
39
40/**
41 * @file
42 * Definition of a snoop filter.
43 */
44
45#ifndef __MEM_SNOOP_FILTER_HH__
46#define __MEM_SNOOP_FILTER_HH__
47
48#include <utility>
49
50#include "base/hashmap.hh"
51#include "mem/packet.hh"
52#include "mem/port.hh"
53#include "mem/qport.hh"
54#include "params/SnoopFilter.hh"
55#include "sim/sim_object.hh"
56#include "sim/system.hh"
57
58/**
59 * This snoop filter keeps track of which connected port has a
60 * particular line of data. It can be queried (through lookup*) on
61 * memory requests from above (reads / writes / ...); and also from
62 * below (snoops). The snoop filter precisely knows about the location
63 * of lines "above" it through a map from cache line address to
64 * sharers/ports. The snoop filter ties into the flows of requests
65 * (when they succeed at the lower interface), regular responses from
66 * below and also responses from sideway's caches (in update*). This
67 * allows the snoop filter to model cache-line residency by snooping
68 * the messages.
69 *
70 * The tracking happens in two fields to be able to distinguish
71 * between in-flight requests (in requested) and already pulled in
72 * lines (in holder). This distinction is used for producing tighter
73 * assertions and tracking request completion. For safety, (requested
74 * | holder) should be notified and the requesting MSHRs will take
75 * care of ordering.
76 *
77 * Overall, some trickery is required because:
78 * (1) snoops are not followed by an ACK, but only evoke a response if
79 *     they need to (hit dirty)
80 * (2) side-channel information is funnelled through direct modifications of
81 *     pkt, instead of proper messages through the bus
82 * (3) there are no clean evict messages telling the snoop filter that a local,
83 *     upper cache dropped a line, making the snoop filter pessimistic for now
84 * (4) ordering: there is no single point of order in the system.  Instead,
85 *     requesting MSHRs track order between local requests and remote snoops
86 */
87class SnoopFilter : public SimObject {
88  public:
89    typedef std::vector<QueuedSlavePort*> SnoopList;
90
91    SnoopFilter (const SnoopFilterParams *p) :
92        SimObject(p), reqLookupResult(cachedLocations.end()), retryItem{0, 0},
93        linesize(p->system->cacheLineSize()), lookupLatency(p->lookup_latency),
94        maxEntryCount(p->max_capacity / p->system->cacheLineSize())
95    {
96    }
97
98    /**
99     * Init a new snoop filter and tell it about all the
100     * slave ports of the enclosing bus.
101     *
102     * @param slave_ports Slave ports that the bus is attached to.
103     */
104    void setSlavePorts(const SnoopList& slave_ports) {
105        localSlavePortIds.resize(slave_ports.size(), InvalidPortID);
106
107        PortID id = 0;
108        for (const auto& p : slave_ports) {
109            // no need to track this port if it is not snooping
110            if (p->isSnooping()) {
111                slavePorts.push_back(p);
112                localSlavePortIds[p->getId()] = id++;
113            }
114        }
115
116        // make sure we can deal with this many ports
117        fatal_if(id > 8 * sizeof(SnoopMask),
118                 "Snoop filter only supports %d snooping ports, got %d\n",
119                 8 * sizeof(SnoopMask), id);
120    }
121
122    /**
123     * Lookup a request (from a slave port) in the snoop filter and
124     * return a list of other slave ports that need forwarding of the
125     * resulting snoops.  Additionally, update the tracking structures
126     * with new request information. Note that the caller must also
127     * call finishRequest once it is known if the request needs to
128     * retry or not.
129     *
130     * @param cpkt          Pointer to the request packet.  Not changed.
131     * @param slave_port    Slave port where the request came from.
132     * @return Pair of a vector of snoop target ports and lookup latency.
133     */
134    std::pair<SnoopList, Cycles> lookupRequest(const Packet* cpkt,
135                                               const SlavePort& slave_port);
136
137    /**
138     * For an un-successful request, revert the change to the snoop
139     * filter. Also take care of erasing any null entries. This method
140     * relies on the result from lookupRequest being stored in
141     * reqLookupResult.
142     *
143     * @param will_retry    This request will retry on this bus / snoop filter
144     * @param cpkt     Request packet, merely for sanity checking
145     */
146    void finishRequest(bool will_retry, const Packet* cpkt);
147
148    /**
149     * Handle an incoming snoop from below (the master port).  These can upgrade the
150     * tracking logic and may also benefit from additional steering thanks to the
151     * snoop filter.
152     * @param cpkt Pointer to const Packet containing the snoop.
153     * @return Pair with a vector of SlavePorts that need snooping and a lookup
154     *         latency.
155     */
156    std::pair<SnoopList, Cycles> lookupSnoop(const Packet* cpkt);
157
158    /**
159     * Let the snoop filter see any snoop responses that turn into request responses
160     * and indicate cache to cache transfers.  These will update the corresponding
161     * state in the filter.
162     *
163     * @param cpkt     Pointer to const Packet holding the snoop response.
164     * @param rsp_port SlavePort that sends the response.
165     * @param req_port SlavePort that made the original request and is the
166     *                 destination of the snoop response.
167     */
168    void updateSnoopResponse(const Packet *cpkt, const SlavePort& rsp_port,
169                             const SlavePort& req_port);
170
171    /**
172     * Pass snoop responses that travel downward through the snoop filter and let
173     * them update the snoop filter state.  No additional routing happens.
174     *
175     * @param cpkt     Pointer to const Packet holding the snoop response.
176     * @param rsp_port SlavePort that sends the response.
177     * @param req_port MasterPort through which the response leaves this cluster.
178     */
179    void updateSnoopForward(const Packet *cpkt, const SlavePort& rsp_port,
180                            const MasterPort& req_port);
181
182    /**
183     * Update the snoop filter with a response from below (outer / other cache,
184     * or memory) and update the tracking information in the snoop filter.
185     *
186     * @param cpkt       Pointer to const Packet holding the snoop response.
187     * @param slave_port SlavePort that made the original request and is the target
188     *                   of this response.
189     */
190    void updateResponse(const Packet *cpkt, const SlavePort& slave_port);
191
192    /**
193     * Simple factory methods for standard return values for lookupRequest
194     */
195    std::pair<SnoopList, Cycles> snoopAll(Cycles latency) const
196    {
197        return std::make_pair(slavePorts, latency);
198    }
199    std::pair<SnoopList, Cycles> snoopSelected(const SnoopList& slave_ports,
200                                               Cycles latency) const
201    {
202        return std::make_pair(slave_ports, latency);
203    }
204    std::pair<SnoopList, Cycles> snoopDown(Cycles latency) const
205    {
206        SnoopList empty;
207        return std::make_pair(empty , latency);
208    }
209
210    virtual void regStats();
211
212  protected:
213    typedef uint64_t SnoopMask;
214   /**
215    * Per cache line item tracking a bitmask of SlavePorts who have an
216    * outstanding request to this line (requested) or already share a cache line
217    * with this address (holder).
218    */
219    struct SnoopItem {
220        SnoopMask requested;
221        SnoopMask holder;
222    };
223    /**
224     * HashMap of SnoopItems indexed by line address
225     */
226    typedef m5::hash_map<Addr, SnoopItem> SnoopFilterCache;
227
228    /**
229     * Convert a single port to a corresponding, one-hot bitmask
230     * @param port SlavePort that should be converted.
231     * @return One-hot bitmask corresponding to the port.
232     */
233    SnoopMask portToMask(const SlavePort& port) const;
234    /**
235     * Converts a bitmask of ports into the corresponing list of ports
236     * @param ports SnoopMask of the requested ports
237     * @return SnoopList containing all the requested SlavePorts
238     */
239    SnoopList maskToPortList(SnoopMask ports) const;
240
241  private:
242
243    /**
244     * Removes snoop filter items which have no requesters and no holders.
245     */
246    void eraseIfNullEntry(SnoopFilterCache::iterator& sf_it);
247    /** Simple hash set of cached addresses. */
248    SnoopFilterCache cachedLocations;
249    /**
250     * Iterator used to store the result from lookupRequest until we
251     * call finishRequest.
252     */
253    SnoopFilterCache::iterator reqLookupResult;
254    /**
255     * Variable to temporarily store value of snoopfilter entry
256     * incase finishRequest needs to undo changes made in lookupRequest
257     * (because of crossbar retry)
258     */
259    SnoopItem retryItem;
260    /** List of all attached snooping slave ports. */
261    SnoopList slavePorts;
262    /** Track the mapping from port ids to the local mask ids. */
263    std::vector<PortID> localSlavePortIds;
264    /** Cache line size. */
265    const unsigned linesize;
266    /** Latency for doing a lookup in the filter */
267    const Cycles lookupLatency;
268    /** Max capacity in terms of cache blocks tracked, for sanity checking */
269    const unsigned maxEntryCount;
270
271    /** Statistics */
272    Stats::Scalar totRequests;
273    Stats::Scalar hitSingleRequests;
274    Stats::Scalar hitMultiRequests;
275
276    Stats::Scalar totSnoops;
277    Stats::Scalar hitSingleSnoops;
278    Stats::Scalar hitMultiSnoops;
279};
280
281inline SnoopFilter::SnoopMask
282SnoopFilter::portToMask(const SlavePort& port) const
283{
284    assert(port.getId() != InvalidPortID);
285    // if this is not a snooping port, return a zero mask
286    return !port.isSnooping() ? 0 :
287        ((SnoopMask)1) << localSlavePortIds[port.getId()];
288}
289
290inline SnoopFilter::SnoopList
291SnoopFilter::maskToPortList(SnoopMask port_mask) const
292{
293    SnoopList res;
294    for (const auto& p : slavePorts)
295        if (port_mask & portToMask(*p))
296            res.push_back(p);
297    return res;
298}
299#endif // __MEM_SNOOP_FILTER_HH__
300