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, 7 insertions, 7 deletions

diff --git a/src/mem/packet.cc b/src/mem/packet.cc
index 289bc81bc..cacfc1d3b 100644
--- a/src/mem/packet.cc
+++ b/src/mem/packet.cc
@@ -83,7 +83,7 @@ MemCmd::commandInfo[] =
     { SET5(IsWrite, NeedsWritable, IsRequest, NeedsResponse, HasData),
             WriteResp, "WriteReq" },
     /* WriteResp */
-    { SET3(IsWrite, NeedsWritable, IsResponse), InvalidCmd, "WriteResp" },
+    { SET2(IsWrite, IsResponse), InvalidCmd, "WriteResp" },
     /* WritebackDirty */
     { SET4(IsWrite, IsRequest, IsEviction, HasData),
             InvalidCmd, "WritebackDirty" },
@@ -117,7 +117,7 @@ MemCmd::commandInfo[] =
            IsRequest, NeedsResponse),
             UpgradeResp, "SCUpgradeReq" },
     /* UpgradeResp */
-    { SET3(NeedsWritable, IsUpgrade, IsResponse),
+    { SET2(IsUpgrade, IsResponse),
             InvalidCmd, "UpgradeResp" },
     /* SCUpgradeFailReq: generates UpgradeFailResp but still gets the data */
     { SET6(IsRead, NeedsWritable, IsInvalidate,
@@ -125,7 +125,7 @@ MemCmd::commandInfo[] =
             UpgradeFailResp, "SCUpgradeFailReq" },
     /* UpgradeFailResp - Behaves like a ReadExReq, but notifies an SC
      * that it has failed, acquires line as Dirty*/
-    { SET4(IsRead, NeedsWritable, IsResponse, HasData),
+    { SET3(IsRead, IsResponse, HasData),
             InvalidCmd, "UpgradeFailResp" },
     /* ReadExReq - Read issues by a cache, always cache-line aligned,
      * and the response is guaranteed to be writeable (exclusive or
@@ -134,7 +134,7 @@ MemCmd::commandInfo[] =
             ReadExResp, "ReadExReq" },
     /* ReadExResp - Response matching a read exclusive, as we check
      * the need for exclusive also on responses */
-    { SET4(IsRead, NeedsWritable, IsResponse, HasData),
+    { SET3(IsRead, IsResponse, HasData),
             InvalidCmd, "ReadExResp" },
     /* ReadCleanReq - Read issued by a cache, always cache-line
      * aligned, and the response is guaranteed to not contain dirty data
@@ -157,13 +157,13 @@ MemCmd::commandInfo[] =
            IsRequest, NeedsResponse, HasData),
             StoreCondResp, "StoreCondFailReq" },
     /* StoreCondResp */
-    { SET4(IsWrite, NeedsWritable, IsLlsc, IsResponse),
+    { SET3(IsWrite, IsLlsc, IsResponse),
             InvalidCmd, "StoreCondResp" },
     /* SwapReq -- for Swap ldstub type operations */
     { SET6(IsRead, IsWrite, NeedsWritable, IsRequest, HasData, NeedsResponse),
         SwapResp, "SwapReq" },
     /* SwapResp -- for Swap ldstub type operations */
-    { SET5(IsRead, IsWrite, NeedsWritable, IsResponse, HasData),
+    { SET4(IsRead, IsWrite, IsResponse, HasData),
             InvalidCmd, "SwapResp" },
     /* IntReq -- for interrupts */
     { SET4(IsWrite, IsRequest, NeedsResponse, HasData),
@@ -190,7 +190,7 @@ MemCmd::commandInfo[] =
     { SET4(IsInvalidate, IsRequest, NeedsWritable, NeedsResponse),
       InvalidateResp, "InvalidateReq" },
     /* Invalidation Response */
-    { SET3(IsInvalidate, IsResponse, NeedsWritable),
+    { SET2(IsInvalidate, IsResponse),
       InvalidCmd, "InvalidateResp" }
 };