Hi All,
I was skimming through the O3+Ruby portion of the current dev repository code,
that attempts to support load-load ordering for a stronger consistency model.
In existing code, L1 cache controller sends a forward_eviction_to_cpu, which in
turn set the hitExternalSnoop flag of a load instruction (provided the load has
not committed yet) through checksnoop() function. The checkViolation() portion
of the code says that in order to be squashed and re-executed, that particular
load instruction has to see another supposedly older load that maps to the same
cache block (the first if () block). The code snippet is shown below:
lsq_unit_impl.hh
checkViolations()
------------
if (inst_eff_addr2 >= ld_eff_addr1 && inst_eff_addr1 <= ld_eff_addr2) {
if (inst->isLoad()) {
// If this load is to the same block as an external snoop
// invalidate that we've observed then the load needs to be
// squashed as it could have newer data
if (ld_inst->hitExternalSnoop) {
if (!memDepViolator ||
ld_inst->seqNum < memDepViolator->seqNum) {
DPRINTF(LSQUnit, "Detected fault with inst [sn:%lli] "
"and [sn:%lli] at address %#x\n",
inst->seqNum, ld_inst->seqNum, ld_eff_addr1);
memDepViolator = ld_inst;
++lsqMemOrderViolation;
return new GenericISA::M5PanicFault(
"Detected fault with inst [sn:%lli] and "
"[sn:%lli] at address %#x\n",
inst->seqNum, ld_inst->seqNum, ld_eff_addr1);
}
}
// Otherwise, mark the load has a possible load violation
// and if we see a snoop before it's commited, we need to squash
ld_inst->possibleLoadViolation = true;
DPRINTF(LSQUnit, "Found possible load violaiton at addr: %#x"
" between instructions [sn:%lli] and [sn:%lli]\n",
inst_eff_addr1, inst->seqNum, ld_inst->seqNum);
} else {
---------------
---------------
In my understanding, if a snoop hits a younger load in lsq before it is
committed, it need to be re-executed without any constraints from
checkViolation() function to maintain stronger consistency. I was talking about
the following simple example:
c0 c1
St B Ld A
St A Ld B
if Ld B in core1 is executed out-of-order and later sees a snoop before commit,
should not we re-execute Ld B without any constraints from checkViolations()
function? Am I missing something completely over here?
Regards,
Dibakar
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