mem: Snoop into caches on uncacheable accesses

This patch takes a last step in fixing issues related to uncacheable
accesses. We do not separate uncacheable memory from uncacheable
devices, and in cases where it is really memory, there are valid
scenarios where we need to snoop since we do not support cache
maintenance instructions (yet). On snooping an uncacheable access we
thus provide data if possible. In essence this makes uncacheable
accesses IO coherent.

The snoop filter is also queried to steer the snoops, but not updated
since the uncacheable accesses do not allocate a block.

src/mem/cache/base.cc

@@ -153,6 +153,17 @@ BaseCache::getSlavePort(const std::string &if_name, PortID idx)
     }
 }
 
+bool
+BaseCache::inRange(Addr addr) const
+{
+    for (const auto& r : addrRanges) {
+        if (r.contains(addr)) {
+            return true;
+       }
+    }
+    return false;
+}
+
 void
 BaseCache::regStats()
 {

src/mem/cache/base.hh

@@ -262,6 +262,16 @@ class BaseCache : public MemObject
      */
     virtual bool isDirty() const = 0;
 
+    /**
+     * Determine if an address is in the ranges covered by this
+     * cache. This is useful to filter snoops.
+     *
+     * @param addr Address to check against
+     *
+     * @return If the address in question is in range
+     */
+    bool inRange(Addr addr) const;
+
     /** Block size of this cache */
     const unsigned blkSize;
 

src/mem/cache/cache_impl.hh

@@ -475,7 +475,6 @@ Cache::recvTimingReq(PacketPtr pkt)
         // responding to the request
         DPRINTF(Cache, "mem inhibited on addr %#llx (%s): not responding\n",
                 pkt->getAddr(), pkt->isSecure() ? "s" : "ns");
-        assert(!pkt->req->isUncacheable());
 
         // if the packet needs exclusive, and the cache that has
         // promised to respond (setting the inhibit flag) is not
@@ -856,7 +855,6 @@ Cache::recvAtomic(PacketPtr pkt)
     promoteWholeLineWrites(pkt);
 
     if (pkt->memInhibitAsserted()) {
-        assert(!pkt->req->isUncacheable());
         // have to invalidate ourselves and any lower caches even if
         // upper cache will be responding
         if (pkt->isInvalidate()) {
@@ -1560,7 +1558,8 @@ Cache::doTimingSupplyResponse(PacketPtr req_pkt, const uint8_t *blk_data,
         // responses)
         pkt = new Packet(req_pkt, false, req_pkt->isRead());
 
-    assert(req_pkt->isInvalidate() || pkt->sharedAsserted());
+    assert(req_pkt->req->isUncacheable() || req_pkt->isInvalidate() ||
+           pkt->sharedAsserted());
     pkt->makeTimingResponse();
     if (pkt->isRead()) {
         pkt->setDataFromBlock(blk_data, blkSize);
@@ -1676,7 +1675,7 @@ Cache::handleSnoop(PacketPtr pkt, CacheBlk *blk, bool is_timing,
         return;
     }
 
-    if (pkt->isRead() && !invalidate) {
+    if (!pkt->req->isUncacheable() && pkt->isRead() && !invalidate) {
         assert(!needs_exclusive);
         pkt->assertShared();
         int bits_to_clear = BlkWritable;
@@ -1699,6 +1698,9 @@ Cache::handleSnoop(PacketPtr pkt, CacheBlk *blk, bool is_timing,
         // will write it back at a later point
         pkt->assertMemInhibit();
         if (have_exclusive) {
+            // in the case of an uncacheable request there is no need
+            // to set the exclusive flag, but since the recipient does
+            // not care there is no harm in doing so
             pkt->setSupplyExclusive();
         }
         if (is_timing) {
@@ -1707,7 +1709,9 @@ Cache::handleSnoop(PacketPtr pkt, CacheBlk *blk, bool is_timing,
             pkt->makeAtomicResponse();
             pkt->setDataFromBlock(blk->data, blkSize);
         }
-    } else if (is_timing && is_deferred) {
+    }
+
+    if (!respond && is_timing && is_deferred) {
         // if it's a deferred timing snoop then we've made a copy of
         // the packet, and so if we're not using that copy to respond
         // then we need to delete it here.
@@ -1735,25 +1739,8 @@ Cache::recvTimingSnoopReq(PacketPtr pkt)
     // Snoops shouldn't happen when bypassing caches
     assert(!system->bypassCaches());
 
-    // check if the packet is for an address range covered by this
-    // cache, partly to not waste time looking for it, but also to
-    // ensure that we only forward the snoop upwards if it is within
-    // our address ranges
-    bool in_range = false;
-    for (AddrRangeList::const_iterator r = addrRanges.begin();
-         r != addrRanges.end(); ++r) {
-        if (r->contains(pkt->getAddr())) {
-            in_range = true;
-            break;
-        }
-    }
-
-    // Note that some deferred snoops don't have requests, since the
-    // original access may have already completed
-    if ((pkt->req && pkt->req->isUncacheable()) ||
-        pkt->cmd == MemCmd::Writeback || !in_range) {
-        //Can't get a hit on an uncacheable address
-        //Revisit this for multi level coherence
+    // no need to snoop writebacks or requests that are not in range
+    if (pkt->cmd == MemCmd::Writeback || !inRange(pkt->getAddr())) {
         return;
     }
 
@@ -1843,9 +1830,8 @@ Cache::recvAtomicSnoop(PacketPtr pkt)
     // Snoops shouldn't happen when bypassing caches
     assert(!system->bypassCaches());
 
-    if (pkt->req->isUncacheable() || pkt->cmd == MemCmd::Writeback) {
-        // Can't get a hit on an uncacheable address
-        // Revisit this for multi level coherence
+    // no need to snoop writebacks or requests that are not in range
+    if (pkt->cmd == MemCmd::Writeback || !inRange(pkt->getAddr())) {
         return 0;
     }
 

src/mem/cache/mshr.cc

@@ -371,6 +371,9 @@ MSHR::handleSnoop(PacketPtr pkt, Counter _order)
 
         if (isPendingDirty()) {
             pkt->assertMemInhibit();
+            // in the case of an uncacheable request there is no need
+            // to set the exclusive flag, but since the recipient does
+            // not care there is no harm in doing so
             pkt->setSupplyExclusive();
         }
 
@@ -380,7 +383,7 @@ MSHR::handleSnoop(PacketPtr pkt, Counter _order)
         }
     }
 
-    if (!pkt->needsExclusive()) {
+    if (!pkt->needsExclusive() && !pkt->req->isUncacheable()) {
         // This transaction will get a read-shared copy, downgrading
         // our copy if we had an exclusive one
         postDowngrade = true;