snoop_filter.hh revision 12429:beefb9f5f551 
1/*
2 * Copyright (c) 2013-2016 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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13 *
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15 * modification, are permitted provided that the following conditions are
16 * met: redistributions of source code must retain the above copyright
17 * notice, this list of conditions and the following disclaimer;
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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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29 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
30 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
31 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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35 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
36 *
37 * Authors: Stephan Diestelhorst
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 <unordered_map>
49#include <utility>
50
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 slave ports
100     * 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 addr          Packet address, merely for sanity checking
145     */
146    void finishRequest(bool will_retry, Addr addr, bool is_secure);
147
148    /**
149     * Handle an incoming snoop from below (the master port). These
150     * can upgrade the tracking logic and may also benefit from
151     * additional steering thanks to the snoop filter.
152     *
153     * @param cpkt Pointer to const Packet containing the snoop.
154     * @return Pair with a vector of SlavePorts that need snooping and a lookup
155     *         latency.
156     */
157    std::pair<SnoopList, Cycles> lookupSnoop(const Packet* cpkt);
158
159    /**
160     * Let the snoop filter see any snoop responses that turn into
161     * request responses and indicate cache to cache transfers. These
162     * will update the corresponding state in the filter.
163     *
164     * @param cpkt     Pointer to const Packet holding the snoop response.
165     * @param rsp_port SlavePort that sends the response.
166     * @param req_port SlavePort that made the original request and is the
167     *                 destination of the snoop response.
168     */
169    void updateSnoopResponse(const Packet *cpkt, const SlavePort& rsp_port,
170                             const SlavePort& req_port);
171
172    /**
173     * Pass snoop responses that travel downward through the snoop
174     * filter and let them update the snoop filter state.  No
175     * additional routing happens.
176     *
177     * @param cpkt     Pointer to const Packet holding the snoop response.
178     * @param rsp_port SlavePort that sends the response.
179     * @param req_port MasterPort through which the response is forwarded.
180     */
181    void updateSnoopForward(const Packet *cpkt, const SlavePort& rsp_port,
182                            const MasterPort& req_port);
183
184    /**
185     * Update the snoop filter with a response from below (outer /
186     * other cache, or memory) and update the tracking information in
187     * the snoop filter.
188     *
189     * @param cpkt       Pointer to const Packet holding the snoop response.
190     * @param slave_port SlavePort that made the original request and
191     *                   is the target of this response.
192     */
193    void updateResponse(const Packet *cpkt, const SlavePort& slave_port);
194
195    virtual void regStats();
196
197  protected:
198
199    /**
200     * The underlying type for the bitmask we use for tracking. This
201     * limits the number of snooping ports supported per crossbar. For
202     * the moment it is an uint64_t to offer maximum
203     * scalability. However, it is possible to use e.g. a uint16_t or
204     * uint32_to slim down the footprint of the hash map (and
205     * ultimately improve the simulation performance).
206     */
207    typedef uint64_t SnoopMask;
208
209    /**
210    * Per cache line item tracking a bitmask of SlavePorts who have an
211    * outstanding request to this line (requested) or already share a
212    * cache line with this address (holder).
213    */
214    struct SnoopItem {
215        SnoopMask requested;
216        SnoopMask holder;
217    };
218    /**
219     * HashMap of SnoopItems indexed by line address
220     */
221    typedef std::unordered_map<Addr, SnoopItem> SnoopFilterCache;
222
223    /**
224     * Simple factory methods for standard return values.
225     */
226    std::pair<SnoopList, Cycles> snoopAll(Cycles latency) const
227    {
228        return std::make_pair(slavePorts, latency);
229    }
230    std::pair<SnoopList, Cycles> snoopSelected(const SnoopList& slave_ports,
231                                               Cycles latency) const
232    {
233        return std::make_pair(slave_ports, latency);
234    }
235    std::pair<SnoopList, Cycles> snoopDown(Cycles latency) const
236    {
237        SnoopList empty;
238        return std::make_pair(empty , latency);
239    }
240
241    /**
242     * Convert a single port to a corresponding, one-hot bitmask
243     * @param port SlavePort that should be converted.
244     * @return One-hot bitmask corresponding to the port.
245     */
246    SnoopMask portToMask(const SlavePort& port) const;
247    /**
248     * Converts a bitmask of ports into the corresponing list of ports
249     * @param ports SnoopMask of the requested ports
250     * @return SnoopList containing all the requested SlavePorts
251     */
252    SnoopList maskToPortList(SnoopMask ports) const;
253
254  private:
255
256    /**
257     * Removes snoop filter items which have no requesters and no holders.
258     */
259    void eraseIfNullEntry(SnoopFilterCache::iterator& sf_it);
260
261    /** Simple hash set of cached addresses. */
262    SnoopFilterCache cachedLocations;
263    /**
264     * Iterator used to store the result from lookupRequest until we
265     * call finishRequest.
266     */
267    SnoopFilterCache::iterator reqLookupResult;
268    /**
269     * Variable to temporarily store value of snoopfilter entry
270     * incase finishRequest needs to undo changes made in lookupRequest
271     * (because of crossbar retry)
272     */
273    SnoopItem retryItem;
274    /** List of all attached snooping slave ports. */
275    SnoopList slavePorts;
276    /** Track the mapping from port ids to the local mask ids. */
277    std::vector<PortID> localSlavePortIds;
278    /** Cache line size. */
279    const unsigned linesize;
280    /** Latency for doing a lookup in the filter */
281    const Cycles lookupLatency;
282    /** Max capacity in terms of cache blocks tracked, for sanity checking */
283    const unsigned maxEntryCount;
284
285    /**
286     * Use the lower bits of the address to keep track of the line status
287     */
288    enum LineStatus {
289        /** block holds data from the secure memory space */
290        LineSecure = 0x01,
291    };
292
293    /** Statistics */
294    Stats::Scalar totRequests;
295    Stats::Scalar hitSingleRequests;
296    Stats::Scalar hitMultiRequests;
297
298    Stats::Scalar totSnoops;
299    Stats::Scalar hitSingleSnoops;
300    Stats::Scalar hitMultiSnoops;
301};
302
303inline SnoopFilter::SnoopMask
304SnoopFilter::portToMask(const SlavePort& port) const
305{
306    assert(port.getId() != InvalidPortID);
307    // if this is not a snooping port, return a zero mask
308    return !port.isSnooping() ? 0 :
309        ((SnoopMask)1) << localSlavePortIds[port.getId()];
310}
311
312inline SnoopFilter::SnoopList
313SnoopFilter::maskToPortList(SnoopMask port_mask) const
314{
315    SnoopList res;
316    for (const auto& p : slavePorts)
317        if (port_mask & portToMask(*p))
318            res.push_back(p);
319    return res;
320}
321
322#endif // __MEM_SNOOP_FILTER_HH__
323