[gem5-dev] changeset in gem5: cpu: o3: lsq: Fix TSO implementation

[Marco Elver]

changeset 45a67d84fd4a in /z/repo/gem5
details: http://repo.gem5.org/gem5?cmd=changeset;node=45a67d84fd4a
description:
        cpu: o3: lsq: Fix TSO implementation
        This patch fixes violation of TSO in the O3CPU, as all loads must be
        ordered with all other loads. In the LQ, if a snoop is observed, all
        subsequent loads need to be squashed if the system is TSO.

        Prior to this patch, the following case could be violated:

         P0         | P1          ;
         MOV [x],mail=/usr/spool/mail/nilay | MOV EAX,[y] ;
         MOV [y],mail=/usr/spool/mail/nilay | MOV EBX,[x] ;

        exists (1:EAX=1 /\ 1:EBX=0) [is a violation]

        The problem was found using litmus [http://diy.inria.fr].

        Committed by: Nilay Vaish <ni...@cs.wisc.edu

diffstat:

 src/cpu/o3/lsq_unit_impl.hh |  12 ++++++++++--
 1 files changed, 10 insertions(+), 2 deletions(-)

diffs (29 lines):

diff -r 4574d5882066 -r 45a67d84fd4a src/cpu/o3/lsq_unit_impl.hh
--- a/src/cpu/o3/lsq_unit_impl.hh       Sun Mar 23 11:12:19 2014 -0400
+++ b/src/cpu/o3/lsq_unit_impl.hh       Tue Mar 25 13:15:04 2014 -0500
@@ -464,6 +464,8 @@
 
     incrLdIdx(load_idx);
 
+    bool force_squash = false;
+
     while (load_idx != loadTail) {
         DynInstPtr ld_inst = loadQueue[load_idx];
 
@@ -476,8 +478,14 @@
         DPRINTF(LSQUnit, "-- inst [sn:%lli] load_addr: %#x to pktAddr:%#x\n",
                     ld_inst->seqNum, load_addr, invalidate_addr);
 
-        if (load_addr == invalidate_addr) {
-            if (ld_inst->possibleLoadViolation()) {
+        if (load_addr == invalidate_addr || force_squash) {
+            if (needsTSO) {
+                // If we have a TSO system, as all loads must be ordered with
+                // all other loads, this load as well as *all* subsequent loads
+                // need to be squashed to prevent possible load reordering.
+                force_squash = true;
+            }
+            if (ld_inst->possibleLoadViolation() || force_squash) {
                 DPRINTF(LSQUnit, "Conflicting load at addr %#x [sn:%lli]\n",
                         pkt->getAddr(), ld_inst->seqNum);
 
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