mem: Only track snooping ports in the snoop filter

This patch changes the tracking of ports in the snoop filter to use
local dense port IDs so that we can have 64 snooping ports (rather
than crossbar slave ports). This is achieved by adding a simple
remapping vector that translates the actal port IDs into the local
slave IDs used in the SnoopMask.

Ultimately this patch allows us to scale to much larger systems
without introducing a hierarchy of crossbars.

src/mem/coherent_xbar.cc

@@ -96,9 +96,6 @@ CoherentXBar::CoherentXBar(const CoherentXBarParams *p)
         snoopRespPorts.push_back(new SnoopRespPort(*bp, *this));
     }
 
-    if (snoopFilter)
-        snoopFilter->setSlavePorts(slavePorts);
-
     clearPortCache();
 }
 
@@ -133,6 +130,11 @@ CoherentXBar::init()
 
     if (snoopPorts.empty())
         warn("CoherentXBar %s has no snooping ports attached!\n", name());
+
+    // inform the snoop filter about the slave ports so it can create
+    // its own internal representation
+    if (snoopFilter)
+        snoopFilter->setSlavePorts(slavePorts);
 }
 
 bool

src/mem/snoop_filter.hh

@@ -96,13 +96,27 @@ class SnoopFilter : public SimObject {
     }
 
     /**
-     * Init a new snoop filter and tell it about all the slave ports of the
-     * enclosing bus.
+     * Init a new snoop filter and tell it about all the
+     * slave ports of the enclosing bus.
      *
-     * @param bus_slave_ports Vector of slave ports that the bus is attached to.
+     * @param slave_ports Slave ports that the bus is attached to.
      */
-    void setSlavePorts(const SnoopList& bus_slave_ports) {
-        slavePorts = bus_slave_ports;
+    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);
     }
 
     /**
@@ -217,12 +231,6 @@ class SnoopFilter : public SimObject {
      * @return One-hot bitmask corresponding to the port.
      */
     SnoopMask portToMask(const SlavePort& port) const;
-    /**
-     * Convert multiple ports to a corresponding bitmask
-     * @param ports SnoopList that should be converted.
-     * @return Bitmask corresponding to the ports in the list.
-     */
-    SnoopMask portListToMask(const SnoopList& ports) const;
     /**
      * Converts a bitmask of ports into the corresponing list of ports
      * @param ports SnoopMask of the requested ports
@@ -249,8 +257,10 @@ class SnoopFilter : public SimObject {
      * (because of crossbar retry)
      */
     SnoopItem retryItem;
-    /** List of all attached slave ports. */
+    /** 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 */
@@ -271,29 +281,19 @@ class SnoopFilter : public SimObject {
 inline SnoopFilter::SnoopMask
 SnoopFilter::portToMask(const SlavePort& port) const
 {
-    unsigned id = (unsigned)port.getId();
-    assert(id != (unsigned)InvalidPortID);
-    assert((int)id < 8 * sizeof(SnoopMask));
-
-    return ((SnoopMask)1) << id;
-}
-
-inline SnoopFilter::SnoopMask
-SnoopFilter::portListToMask(const SnoopList& ports) const
-{
-    SnoopMask m = 0;
-    for (auto port = ports.begin(); port != ports.end(); ++port)
-        m |= portToMask(**port);
-    return m;
+    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 (auto port = slavePorts.begin(); port != slavePorts.end(); ++port)
-        if (port_mask & portToMask(**port))
-            res.push_back(*port);
+    for (const auto& p : slavePorts)
+        if (port_mask & portToMask(*p))
+            res.push_back(p);
     return res;
 }
 #endif // __MEM_SNOOP_FILTER_HH__