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.

1 files changed, 10 insertions, 0 deletions

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,