cpu: fixed how O3 CPU executes an exit system call

When a thread executed an exit syscall in SE mode, the thread context
was removed immediately in the same cycle, which left inflight squash
operations and trap event incomplete. The problem happened when a new
thread was assigned to the CPU later. The new thread started with some
incomplete transactions of the previous thread (e.g., squashing). This
problem could cause incorrect execution flow for the new thread (i.e.,
pc was not reset properly at the exit point), deadlock (i.e., some
stage-to-stage signals were not reset) and incorrect rename map between
logical and physical registers.

This patch adds a new state called 'Halting' to the thread context and
defers removing thread context from a CPU until a trap event initiated
by an exit syscall execution is processed. This patch also makes sure
that the removal of a thread context happens after all inflight
transactions of the to-be-removed thread in the pipeline complete.

Change-Id: If7ef1462fb8864e22b45371ee7ae67e2a5ad38b8
Reviewed-on: https://gem5-review.googlesource.com/c/8184
Reviewed-by: Giacomo Gabrielli <giacomo.gabrielli@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/sim/eventq.hh b/src/sim/eventq.hh
index 6e8e63338..895f69424 100644
--- a/src/sim/eventq.hh
+++ b/src/sim/eventq.hh
@@ -161,6 +161,9 @@ class EventBase
     /// (such as writebacks).
     static const Priority CPU_Tick_Pri =                50;
 
+    /// If we want to exit a thread in a CPU, it comes after CPU_Tick_Pri
+    static const Priority CPU_Exit_Pri =                64;
+
     /// Statistics events (dump, reset, etc.) come after
     /// everything else, but before exit.
     static const Priority Stat_Event_Pri =              90;