mem: Add ReadCleanReq and ReadSharedReq packets

This patch adds two new read requests packets:

ReadCleanReq - For a cache to explicitly request clean data. The
response is thus exclusive or shared, but not owned or modified. The
read-only caches (see previous patch) use this request type to ensure
they do not get dirty data.

ReadSharedReq - We add this to distinguish cache read requests from
those issued by other masters, such as devices and CPUs. Thus, devices
use ReadReq, and caches use ReadCleanReq, ReadExReq, or
ReadSharedReq. For the latter, the response can be any state, shared,
exclusive, owned or even modified.

Both ReadCleanReq and ReadSharedReq re-use the normal ReadResp. The
two transactions are aligned with the emerging cache-coherent TLM
standard and the AMBA nomenclature.

With this change, the normal ReadReq should never be used by a cache,
and is reserved for the actual (non-caching) masters in the system. We
thus have a way of identifying if a request came from a cache or
not. The introduction of ReadSharedReq thus removes the need for the
current isTopLevel hack, and also allows us to stop relying on
checking the packet size to determine if the source is a cache or
not. This is fixed in follow-on patches.

src/mem/cache/base.cc

@@ -190,7 +190,8 @@ BaseCache::regStats()
 // to change the subset of commands that are considered "demand" vs
 // "non-demand"
 #define SUM_DEMAND(s) \
-    (s[MemCmd::ReadReq] + s[MemCmd::WriteReq] + s[MemCmd::ReadExReq])
+    (s[MemCmd::ReadReq] + s[MemCmd::WriteReq] + \
+     s[MemCmd::ReadExReq] + s[MemCmd::ReadCleanReq] + s[MemCmd::ReadSharedReq])
 
 // should writebacks be included here?  prior code was inconsistent...
 #define SUM_NON_DEMAND(s) \

src/mem/cache/cache_impl.hh

@@ -184,6 +184,10 @@ Cache::satisfyCpuSideRequest(PacketPtr pkt, CacheBlk *blk,
             // special handling for coherent block requests from
             // upper-level caches
             if (pkt->needsExclusive()) {
+                // sanity check
+                assert(pkt->cmd == MemCmd::ReadExReq ||
+                       pkt->cmd == MemCmd::SCUpgradeFailReq);
+
                 // if we have a dirty copy, make sure the recipient
                 // keeps it marked dirty
                 if (blk->isDirty()) {
@@ -193,8 +197,9 @@ Cache::satisfyCpuSideRequest(PacketPtr pkt, CacheBlk *blk,
                 if (blk != tempBlock)
                     tags->invalidate(blk);
                 blk->invalidate();
-            } else if (blk->isWritable() && !pending_downgrade
-                      && !pkt->sharedAsserted() && !pkt->req->isInstFetch()) {
+            } else if (blk->isWritable() && !pending_downgrade &&
+                       !pkt->sharedAsserted() &&
+                       pkt->cmd != MemCmd::ReadCleanReq) {
                 // we can give the requester an exclusive copy (by not
                 // asserting shared line) on a read request if:
                 // - we have an exclusive copy at this level (& below)
@@ -901,7 +906,8 @@ Cache::getBusPacket(PacketPtr cpu_pkt, CacheBlk *blk,
         cmd = cpu_pkt->cmd;
     } else {
         // block is invalid
-        cmd = needsExclusive ? MemCmd::ReadExReq : MemCmd::ReadReq;
+        cmd = needsExclusive ? MemCmd::ReadExReq :
+            (isReadOnly ? MemCmd::ReadCleanReq : MemCmd::ReadSharedReq);
     }
     PacketPtr pkt = new Packet(cpu_pkt->req, cmd, blkSize);
 

src/mem/packet.cc

@@ -71,7 +71,8 @@ MemCmd::commandInfo[] =
 {
     /* InvalidCmd */
     { 0, InvalidCmd, "InvalidCmd" },
-    /* ReadReq */
+    /* ReadReq - Read issued by a non-caching agent such as a CPU or
+     * device, with no restrictions on alignment. */
     { SET3(IsRead, IsRequest, NeedsResponse), ReadResp, "ReadReq" },
     /* ReadResp */
     { SET3(IsRead, IsResponse, HasData), InvalidCmd, "ReadResp" },
@@ -125,12 +126,23 @@ MemCmd::commandInfo[] =
      * that it has failed, acquires line as Dirty*/
     { SET4(IsRead, NeedsExclusive, IsResponse, HasData),
             InvalidCmd, "UpgradeFailResp" },
-    /* ReadExReq */
+    /* ReadExReq - Read issues by a cache, always cache-line aligned,
+     * and the response is guaranteed to be writeable (exclusive or
+     * even modified) */
     { SET5(IsRead, NeedsExclusive, IsInvalidate, IsRequest, NeedsResponse),
             ReadExResp, "ReadExReq" },
-    /* ReadExResp */
+    /* ReadExResp - Response matching a read exclusive, as we check
+     * the need for exclusive also on responses */
     { SET4(IsRead, NeedsExclusive, IsResponse, HasData),
             InvalidCmd, "ReadExResp" },
+    /* ReadCleanReq - Read issued by a cache, always cache-line
+     * aligned, and the response is guaranteed to not contain dirty data
+     * (exclusive or shared).*/
+    { SET3(IsRead, IsRequest, NeedsResponse), ReadResp, "ReadCleanReq" },
+    /* ReadSharedReq - Read issued by a cache, always cache-line
+     * aligned, response is shared, possibly exclusive, owned or even
+     * modified. */
+    { SET3(IsRead, IsRequest, NeedsResponse), ReadResp, "ReadSharedReq" },
     /* LoadLockedReq: note that we use plain ReadResp as response, so that
      *                we can also use ReadRespWithInvalidate when needed */
     { SET4(IsRead, IsLlsc, IsRequest, NeedsResponse),

src/mem/packet.hh

@@ -101,6 +101,8 @@ class MemCmd
         UpgradeFailResp,        // Valid for SCUpgradeReq only
         ReadExReq,
         ReadExResp,
+        ReadCleanReq,
+        ReadSharedReq,
         LoadLockedReq,
         StoreCondReq,
         StoreCondFailReq,       // Failed StoreCondReq in MSHR (never sent)