mem: Add an option to perform clean writebacks from caches

This patch adds the necessary commands and cache functionality to
allow clean writebacks. This functionality is crucial, especially when
having exclusive (victim) caches. For example, if read-only L1
instruction caches are not sending clean writebacks, there will never
be any spills from the L1 to the L2. At the moment the cache model
defaults to not sending clean writebacks, and this should possibly be
re-evaluated.

The implementation of clean writebacks relies on a new packet command
WritebackClean, which acts much like a Writeback (renamed
WritebackDirty), and also much like a CleanEvict. On eviction of a
clean block the cache either sends a clean evict, or a clean
writeback, and if any copies are still cached upstream the clean
evict/writeback is dropped. Similarly, if a clean evict/writeback
reaches a cache where there are outstanding MSHRs for the block, the
packet is dropped. In the typical case though, the clean writeback
allocates a block in the downstream cache, and marks it writable if
the evicted block was writable.

The patch changes the O3_ARM_v7a L1 cache configuration and the
default L1 caches in config/common/Caches.py

1 files changed, 9 insertions, 0 deletions

diff --git a/src/mem/cache/cache.hh b/src/mem/cache/cache.hh
index 6da837003..eb40ddb18 100644
--- a/src/mem/cache/cache.hh
+++ b/src/mem/cache/cache.hh
@@ -202,6 +202,15 @@ class Cache : public BaseCache
      */
     const Enums::Clusivity clusivity;
 
+     /**
+     * Determine if clean lines should be written back or not. In
+     * cases where a downstream cache is mostly inclusive we likely
+     * want it to act as a victim cache also for lines that have not
+     * been modified. Hence, we cannot simply drop the line (or send a
+     * clean evict), but rather need to send the actual data.
+     */
+    const bool writebackClean;
+
     /**
      * Upstream caches need this packet until true is returned, so
      * hold it for deletion until a subsequent call