sim: Don't quiesce UDelayEvents with 0 latency

ARM uses UDelayEvents to emulate kernel __*udelay functions and speed up
simulation. UDelayEvents call Pseudoinst::quiesceNs to quiesce the system for
a specified delay. Changeset 10341:0b4d10f53c2d introduced the requirement
that any quiesce process that is started must also be completed by scheduling
an EndQuiesceEvent. This change causes the CPU to hang if an IsQuiesce
instruction is executed, but the corresponding EndQuiesceEvent is not
scheduled.

Changeset 11058:d0934b57735a introduces a fix for uses of PseudoInst::quiesce*
that would conditionally execute the EndQuiesceEvent. ARM UDelayEvents specify
quiesce period of 0 ns (src/arch/arm/linux/system.cc), so changeset 11058
causes these events to now execute full quiesce processes, greatly increasing
the total instructions executed in kernel delay loops and slowing simulation.

This patch updates the UDelayEvent to conditionally execute
PseudoInst::quiesceNs (**a quiesce operation**) only if the specified
delay is >0 ns. The result is ARM delay loops no longer execute instructions
for quiesce handling, and regression time returns to normal.

1 files changed, 7 insertions, 1 deletions

diff --git a/src/kern/freebsd/events.cc b/src/kern/freebsd/events.cc
index 2f4ceb056..15954d672 100644
--- a/src/kern/freebsd/events.cc
+++ b/src/kern/freebsd/events.cc
@@ -65,7 +65,13 @@ UDelayEvent::process(ThreadContext *tc)
 
     SkipFuncEvent::process(tc);
 
-    PseudoInst::quiesceNs(tc, time);
+    // Currently, only ARM full-system simulation uses UDelayEvents to skip
+    // __delay and __loop_delay functions. One form involves setting quiesce
+    // time to 0 with the assumption that quiesce will not happen. To avoid
+    // the quiesce handling in this case, only execute the quiesce if time > 0.
+    if (time > 0) {
+        PseudoInst::quiesceNs(tc, time);
+    }
 }
 
 } // namespace FreeBSD