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, 5 insertions, 14 deletions

diff --git a/src/cpu/o3/iew.hh b/src/cpu/o3/iew.hh
index dcc8ecf82..5adf32752 100644
--- a/src/cpu/o3/iew.hh
+++ b/src/cpu/o3/iew.hh
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2010 ARM Limited
+ * Copyright (c) 2010-2012 ARM Limited
  * All rights reserved
  *
  * The license below extends only to copyright in the software and shall
@@ -150,21 +150,15 @@ class DefaultIEW
     /** Sets pointer to the scoreboard. */
     void setScoreboard(Scoreboard *sb_ptr);
 
-    /** Drains IEW stage. */
-    bool drain();
+    /** Perform sanity checks after a drain. */
+    void drainSanityCheck() const;
 
-    /** Resumes execution after a drain. */
-    void resume();
-
-    /** Completes switch out of IEW stage. */
-    void switchOut();
+    /** Has the stage drained? */
+    bool isDrained() const;
 
     /** Takes over from another CPU's thread. */
     void takeOverFrom();
 
-    /** Returns if IEW is switched out. */
-    bool isSwitchedOut() { return switchedOut; }
-
     /** Squashes instructions in IEW for a specific thread. */
     void squash(ThreadID tid);
 
@@ -470,9 +464,6 @@ class DefaultIEW
     /** Maximum size of the skid buffer. */
     unsigned skidBufferMax;
 
-    /** Is this stage switched out. */
-    bool switchedOut;
-
     /** Stat for total number of idle cycles. */
     Stats::Scalar iewIdleCycles;
     /** Stat for total number of squashing cycles. */