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37 * Authors: Stephan Diestelhorst <stephan.diestelhorst@arm.com>
42 * Definition of a snoop filter.
45 #ifndef __MEM_SNOOP_FILTER_HH__
46 #define __MEM_SNOOP_FILTER_HH__
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"
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
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
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
87 class SnoopFilter : public SimObject {
89 typedef std::vector<QueuedSlavePort*> SnoopList;
91 SnoopFilter (const SnoopFilterParams *p) :
92 SimObject(p), reqLookupResult(cachedLocations.end()), retryItem{0, 0},
93 linesize(p->system->cacheLineSize()), lookupLatency(p->lookup_latency)
98 * Init a new snoop filter and tell it about all the slave ports of the
101 * @param bus_slave_ports Vector of slave ports that the bus is attached to.
103 void setSlavePorts(const SnoopList& bus_slave_ports) {
104 slavePorts = bus_slave_ports;
108 * Lookup a request (from a slave port) in the snoop filter and
109 * return a list of other slave ports that need forwarding of the
110 * resulting snoops. Additionally, update the tracking structures
111 * with new request information. Note that the caller must also
112 * call finishRequest once it is known if the request needs to
115 * @param cpkt Pointer to the request packet. Not changed.
116 * @param slave_port Slave port where the request came from.
117 * @return Pair of a vector of snoop target ports and lookup latency.
119 std::pair<SnoopList, Cycles> lookupRequest(const Packet* cpkt,
120 const SlavePort& slave_port);
123 * For an un-successful request, revert the change to the snoop
124 * filter. Also take care of erasing any null entries. This method
125 * relies on the result from lookupRequest being stored in
128 * @param will_retry This request will retry on this bus / snoop filter
129 * @param cpkt Request packet, merely for sanity checking
131 void finishRequest(bool will_retry, const Packet* cpkt);
134 * Handle an incoming snoop from below (the master port). These can upgrade the
135 * tracking logic and may also benefit from additional steering thanks to the
137 * @param cpkt Pointer to const Packet containing the snoop.
138 * @return Pair with a vector of SlavePorts that need snooping and a lookup
141 std::pair<SnoopList, Cycles> lookupSnoop(const Packet* cpkt);
144 * Let the snoop filter see any snoop responses that turn into request responses
145 * and indicate cache to cache transfers. These will update the corresponding
146 * state in the filter.
148 * @param cpkt Pointer to const Packet holding the snoop response.
149 * @param rsp_port SlavePort that sends the response.
150 * @param req_port SlavePort that made the original request and is the
151 * destination of the snoop response.
153 void updateSnoopResponse(const Packet *cpkt, const SlavePort& rsp_port,
154 const SlavePort& req_port);
157 * Pass snoop responses that travel downward through the snoop filter and let
158 * them update the snoop filter state. No additional routing happens.
160 * @param cpkt Pointer to const Packet holding the snoop response.
161 * @param rsp_port SlavePort that sends the response.
162 * @param req_port MasterPort through which the response leaves this cluster.
164 void updateSnoopForward(const Packet *cpkt, const SlavePort& rsp_port,
165 const MasterPort& req_port);
168 * Update the snoop filter with a response from below (outer / other cache,
169 * or memory) and update the tracking information in the snoop filter.
171 * @param cpkt Pointer to const Packet holding the snoop response.
172 * @param slave_port SlavePort that made the original request and is the target
175 void updateResponse(const Packet *cpkt, const SlavePort& slave_port);
178 * Simple factory methods for standard return values for lookupRequest
180 std::pair<SnoopList, Cycles> snoopAll(Cycles latency) const
182 return std::make_pair(slavePorts, latency);
184 std::pair<SnoopList, Cycles> snoopSelected(const SnoopList& slave_ports,
185 Cycles latency) const
187 return std::make_pair(slave_ports, latency);
189 std::pair<SnoopList, Cycles> snoopDown(Cycles latency) const
192 return std::make_pair(empty , latency);
195 virtual void regStats();
198 typedef uint64_t SnoopMask;
200 * Per cache line item tracking a bitmask of SlavePorts who have an
201 * outstanding request to this line (requested) or already share a cache line
202 * with this address (holder).
209 * HashMap of SnoopItems indexed by line address
211 typedef m5::hash_map<Addr, SnoopItem> SnoopFilterCache;
214 * Convert a single port to a corresponding, one-hot bitmask
215 * @param port SlavePort that should be converted.
216 * @return One-hot bitmask corresponding to the port.
218 SnoopMask portToMask(const SlavePort& port) const;
220 * Convert multiple ports to a corresponding bitmask
221 * @param ports SnoopList that should be converted.
222 * @return Bitmask corresponding to the ports in the list.
224 SnoopMask portListToMask(const SnoopList& ports) const;
226 * Converts a bitmask of ports into the corresponing list of ports
227 * @param ports SnoopMask of the requested ports
228 * @return SnoopList containing all the requested SlavePorts
230 SnoopList maskToPortList(SnoopMask ports) const;
235 * Removes snoop filter items which have no requesters and no holders.
237 void eraseIfNullEntry(SnoopFilterCache::iterator& sf_it);
238 /** Simple hash set of cached addresses. */
239 SnoopFilterCache cachedLocations;
241 * Iterator used to store the result from lookupRequest until we
242 * call finishRequest.
244 SnoopFilterCache::iterator reqLookupResult;
246 * Variable to temporarily store value of snoopfilter entry
247 * incase finishRequest needs to undo changes made in lookupRequest
248 * (because of crossbar retry)
251 /** List of all attached slave ports. */
252 SnoopList slavePorts;
253 /** Cache line size. */
254 const unsigned linesize;
255 /** Latency for doing a lookup in the filter */
256 const Cycles lookupLatency;
259 Stats::Scalar totRequests;
260 Stats::Scalar hitSingleRequests;
261 Stats::Scalar hitMultiRequests;
263 Stats::Scalar totSnoops;
264 Stats::Scalar hitSingleSnoops;
265 Stats::Scalar hitMultiSnoops;
268 inline SnoopFilter::SnoopMask
269 SnoopFilter::portToMask(const SlavePort& port) const
271 unsigned id = (unsigned)port.getId();
272 assert(id != (unsigned)InvalidPortID);
273 assert((int)id < 8 * sizeof(SnoopMask));
275 return ((SnoopMask)1) << id;
278 inline SnoopFilter::SnoopMask
279 SnoopFilter::portListToMask(const SnoopList& ports) const
282 for (auto port = ports.begin(); port != ports.end(); ++port)
283 m |= portToMask(**port);
287 inline SnoopFilter::SnoopList
288 SnoopFilter::maskToPortList(SnoopMask port_mask) const
291 for (auto port = slavePorts.begin(); port != slavePorts.end(); ++port)
292 if (port_mask & portToMask(**port))
293 res.push_back(*port);
296 #endif // __MEM_SNOOP_FILTER_HH__