cpu: Rewrite O3 draining to avoid stopping in microcode

Previously, the O3 CPU could stop in the middle of a microcode
sequence. This patch makes sure that the pipeline stops when it has
committed a normal instruction or exited from a microcode
sequence. Additionally, it makes sure that the pipeline has no
instructions in flight when it is drained, which should make draining
more robust.

Draining is controlled in the commit stage, which checks if the next
PC after a committed instruction is in microcode. If this isn't the
case, it requests a squash of all instructions after that the
instruction that just committed and immediately signals a drain stall
to the fetch stage. The CPU then continues to execute until the
pipeline and all associated buffers are empty.

1 files changed, 7 insertions, 7 deletions

diff --git a/src/cpu/o3/commit.hh b/src/cpu/o3/commit.hh
index 2d8d88b21..c76d6c1d0 100644
--- a/src/cpu/o3/commit.hh
+++ b/src/cpu/o3/commit.hh
@@ -199,13 +199,16 @@ class DefaultCommit
     void startupStage();
 
     /** Initializes the draining of commit. */
-    bool drain();
+    void drain();
 
     /** Resumes execution after draining. */
-    void resume();
+    void drainResume();
 
-    /** Completes the switch out of commit. */
-    void switchOut();
+    /** Perform sanity checks after a drain. */
+    void drainSanityCheck() const;
+
+    /** Has the stage drained? */
+    bool isDrained() const;
 
     /** Takes over from another CPU's thread. */
     void takeOverFrom();
@@ -438,9 +441,6 @@ class DefaultCommit
     /** Is a drain pending. */
     bool drainPending;
 
-    /** Is commit switched out. */
-    bool switchedOut;
-
     /** The latency to handle a trap.  Used when scheduling trap
      * squash event.
      */