mem: Allow read-only caches and check compliance

This patch adds a parameter to the BaseCache to enable a read-only
cache, for example for the instruction cache, or table-walker cache
(not for x86). A number of checks are put in place in the code to
ensure a read-only cache does not end up with dirty data.

A follow-on patch adds suitable read requests to allow a read-only
cache to explicitly ask for clean data.

4 files changed, 32 insertions, 1 deletions

diff --git a/src/mem/cache/BaseCache.py b/src/mem/cache/BaseCache.py
index fdb41bf75..4d6766456 100644
--- a/src/mem/cache/BaseCache.py
+++ b/src/mem/cache/BaseCache.py
@@ -65,6 +65,7 @@ class BaseCache(MemObject):
     forward_snoops = Param.Bool(True,
         "Forward snoops from mem side to cpu side")
     is_top_level = Param.Bool(False, "Is this cache at the top level (e.g. L1)")
+    is_read_only = Param.Bool(False, "Is this cache read only (e.g. inst)")
 
     prefetcher = Param.BasePrefetcher(NULL,"Prefetcher attached to cache")
     prefetch_on_access = Param.Bool(False,
diff --git a/src/mem/cache/base.cc b/src/mem/cache/base.cc
index c2068496c..af504d9bc 100644
--- a/src/mem/cache/base.cc
+++ b/src/mem/cache/base.cc
@@ -79,6 +79,7 @@ BaseCache::BaseCache(const Params *p)
       numTarget(p->tgts_per_mshr),
       forwardSnoops(p->forward_snoops),
       isTopLevel(p->is_top_level),
+      isReadOnly(p->is_read_only),
       blocked(0),
       order(0),
       noTargetMSHR(NULL),
diff --git a/src/mem/cache/base.hh b/src/mem/cache/base.hh
index aaf0ea691..d2cb11f33 100644
--- a/src/mem/cache/base.hh
+++ b/src/mem/cache/base.hh
@@ -310,6 +310,14 @@ class BaseCache : public MemObject
     const bool isTopLevel;
 
     /**
+     * Is this cache read only, for example the instruction cache, or
+     * table-walker cache. A cache that is read only should never see
+     * any writes, and should never get any dirty data (and hence
+     * never have to do any writebacks).
+     */
+    const bool isReadOnly;
+
+    /**
      * Bit vector of the blocking reasons for the access path.
      * @sa #BlockedCause
      */
@@ -516,6 +524,8 @@ class BaseCache : public MemObject
 
     MSHR *allocateWriteBuffer(PacketPtr pkt, Tick time, bool requestBus)
     {
+        // should only see clean evictions in a read-only cache
+        assert(!isReadOnly || pkt->cmd == MemCmd::CleanEvict);
         assert(pkt->isWrite() && !pkt->isRead());
         return allocateBufferInternal(&writeBuffer,
                                       blockAlign(pkt->getAddr()), blkSize,
diff --git a/src/mem/cache/cache_impl.hh b/src/mem/cache/cache_impl.hh
index 117596d9b..5a9205894 100644
--- a/src/mem/cache/cache_impl.hh
+++ b/src/mem/cache/cache_impl.hh
@@ -299,8 +299,13 @@ Cache::access(PacketPtr pkt, CacheBlk *&blk, Cycles &lat,
     // sanity check
     assert(pkt->isRequest());
 
+    chatty_assert(!(isReadOnly && pkt->isWrite()),
+                  "Should never see a write in a read-only cache %s\n",
+                  name());
+
     DPRINTF(Cache, "%s for %s addr %#llx size %d\n", __func__,
             pkt->cmdString(), pkt->getAddr(), pkt->getSize());
+
     if (pkt->req->isUncacheable()) {
         DPRINTF(Cache, "%s%s addr %#llx uncacheable\n", pkt->cmdString(),
                 pkt->req->isInstFetch() ? " (ifetch)" : "",
@@ -1419,6 +1424,7 @@ Cache::recvTimingResp(PacketPtr pkt)
 PacketPtr
 Cache::writebackBlk(CacheBlk *blk)
 {
+    chatty_assert(!isReadOnly, "Writeback from read-only cache");
     assert(blk && blk->isValid() && blk->isDirty());
 
     writebacks[Request::wbMasterId]++;
@@ -1627,7 +1633,12 @@ Cache::handleFill(PacketPtr pkt, CacheBlk *blk, PacketList &writebacks)
     blk->status |= BlkValid | BlkReadable;
 
     if (!pkt->sharedAsserted()) {
+        // we could get non-shared responses from memory (rather than
+        // a cache) even in a read-only cache, note that we set this
+        // bit even for a read-only cache as we use it to represent
+        // the exclusive state
         blk->status |= BlkWritable;
+
         // If we got this via cache-to-cache transfer (i.e., from a
         // cache that was an owner) and took away that owner's copy,
         // then we need to write it back.  Normally this happens
@@ -1635,8 +1646,12 @@ Cache::handleFill(PacketPtr pkt, CacheBlk *blk, PacketList &writebacks)
         // there are cases (such as failed store conditionals or
         // compare-and-swaps) where we'll demand an exclusive copy but
         // end up not writing it.
-        if (pkt->memInhibitAsserted())
+        if (pkt->memInhibitAsserted()) {
             blk->status |= BlkDirty;
+
+            chatty_assert(!isReadOnly, "Should never see dirty snoop response "
+                          "in read-only cache %s\n", name());
+        }
     }
 
     DPRINTF(Cache, "Block addr %#llx (%s) moving from state %x to %s\n",
@@ -1785,6 +1800,10 @@ Cache::handleSnoop(PacketPtr pkt, CacheBlk *blk, bool is_timing,
                pkt->getAddr(), pkt->getSize(), blk->print());
     }
 
+    chatty_assert(!(isReadOnly && blk->isDirty()),
+                  "Should never have a dirty block in a read-only cache %s\n",
+                  name());
+
     // we may end up modifying both the block state and the packet (if
     // we respond in atomic mode), so just figure out what to do now
     // and then do it later.  If we find dirty data while snooping for a