mem: Do not rely on the NeedsWritable flag for responses

This patch removes the NeedsWritable flag for all responses, as it is
really only the request that needs a writable response. The response,
on the other hand, should in these cases always provide the line in a
writable state, as indicated by the hasSharers flag not being set.

When we send requests that has NeedsWritable set, the response will
always have the hasSharers flag not set. Additionally, there are cases
where the request did not have NeedsWritable set, and we still get a
writable response with the hasSharers flag not set. This never happens
on snoops, but is used by downstream caches to pass ownership
upstream.

As part of this patch, the affected response types are updated, and
the snoop filter is similarly modified to check only the hasSharers
flag (as it should). A sanity check is also added to the packet class,
asserting that we never look at the NeedsWritable flag for responses.

No regressions are affected.

1 files changed, 9 insertions, 1 deletions

diff --git a/src/mem/packet.hh b/src/mem/packet.hh
index 94508f697..32c4cf631 100644
--- a/src/mem/packet.hh
+++ b/src/mem/packet.hh
@@ -502,7 +502,15 @@ class Packet : public Printable
     bool isUpgrade()  const          { return cmd.isUpgrade(); }
     bool isRequest() const           { return cmd.isRequest(); }
     bool isResponse() const          { return cmd.isResponse(); }
-    bool needsWritable() const       { return cmd.needsWritable(); }
+    bool needsWritable() const
+    {
+        // we should never check if a response needsWritable, the
+        // request has this flag, and for a response we should rather
+        // look at the hasSharers flag (if not set, the response is to
+        // be considered writable)
+        assert(isRequest());
+        return cmd.needsWritable();
+    }
     bool needsResponse() const       { return cmd.needsResponse(); }
     bool isInvalidate() const        { return cmd.isInvalidate(); }
     bool isEviction() const          { return cmd.isEviction(); }