Guarantee probe forward progress during LR storm

src/main/scala/rocket/DCache.scala

@@ -198,15 +198,15 @@ class DCacheModule(outer: DCache) extends HellaCacheModule(outer) {
   val s2_lr = Bool(usingAtomics) && s2_req.cmd === M_XLR
   val s2_sc = Bool(usingAtomics) && s2_req.cmd === M_XSC
   val lrscCount = Reg(init=UInt(0))
-  val lrscValid = lrscCount > 0
+  val lrscValid = lrscCount > lrscBackoff
   val lrscAddr = Reg(UInt())
   val s2_sc_fail = s2_sc && !(lrscValid && lrscAddr === (s2_req.addr >> blockOffBits))
   when (s2_valid_hit && s2_lr) {
     lrscCount := lrscCycles - 1
     lrscAddr := s2_req.addr >> blockOffBits
   }
-  when (lrscValid) { lrscCount := lrscCount - 1 }
-  when ((s2_valid_masked && lrscValid) || io.cpu.invalidate_lr) { lrscCount := 0 }
+  when (lrscCount > 0) { lrscCount := lrscCount - 1 }
+  when ((s2_valid_masked && lrscCount > 0) || io.cpu.invalidate_lr) { lrscCount := 0 }
 
   // pending store buffer
   val pstore1_cmd = RegEnable(s1_req.cmd, s1_valid_not_nacked && s1_write)

src/main/scala/rocket/HellaCache.scala

@@ -58,6 +58,7 @@ trait HasL1HellaCacheParameters extends HasL1CacheParameters with HasCoreParamet
   def encDataBits = code.width(coreDataBits)
   def encRowBits = encDataBits*rowWords
   def lrscCycles = 32 // ISA requires 16-insn LRSC sequences to succeed
+  def lrscBackoff = 3 // disallow LRSC reacquisition briefly
   def nIOMSHRs = cacheParams.nMMIOs
   def maxUncachedInFlight = cacheParams.nMMIOs
   def dataScratchpadSize = cacheParams.dataScratchpadBytes

src/main/scala/rocket/NBDcache.scala

@@ -794,18 +794,18 @@ class NonBlockingDCacheModule(outer: NonBlockingDCache) extends HellaCacheModule
 
   // load-reserved/store-conditional
   val lrsc_count = Reg(init=UInt(0))
-  val lrsc_valid = lrsc_count.orR
+  val lrsc_valid = lrsc_count > lrscBackoff
   val lrsc_addr = Reg(UInt())
   val (s2_lr, s2_sc) = (s2_req.cmd === M_XLR, s2_req.cmd === M_XSC)
   val s2_lrsc_addr_match = lrsc_valid && lrsc_addr === (s2_req.addr >> blockOffBits)
   val s2_sc_fail = s2_sc && !s2_lrsc_addr_match
-  when (lrsc_valid) { lrsc_count := lrsc_count - 1 }
+  when (lrsc_count > 0) { lrsc_count := lrsc_count - 1 }
   when (s2_valid_masked && s2_hit || s2_replay) {
     when (s2_lr) {
       lrsc_count := lrscCycles - 1
       lrsc_addr := s2_req.addr >> blockOffBits
     }
-    when (lrsc_valid) {
+    when (lrsc_count > 0) {
       lrsc_count := 0
     }
   }