sim: add support for checkpoint downgrading

This commit supports the use case of transitioning tags and their
associated checkpoint rewrites out of use for whatever reason.  Just
replace the upgrader() method with a downgrader() method that performs
the appropriate inverse operation.

The tag name is still used, but only in this negative, 'zombie' state,
as it will be removed from the tags in the checkpoint and gem5 binary.

Change-Id: If9d26cccfe8449e026762b1a72f0c2ae5a9cf2d7
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

1 files changed, 38 insertions, 17 deletions

diff --git a/util/cpt_upgrader.py b/util/cpt_upgrader.py
index 5102fd5a2..b4e4beab2 100755
--- a/util/cpt_upgrader.py
+++ b/util/cpt_upgrader.py
@@ -1,6 +1,6 @@
 #!/usr/bin/env python2
 
-# Copyright (c) 2012-2013,2015 ARM Limited
+# Copyright (c) 2012-2013,2015-2016 ARM Limited
 # All rights reserved
 #
 # The license below extends only to copyright in the software and shall
@@ -57,6 +57,13 @@
 # be isa specific the method can verify the isa and use regexes to find the
 # correct sections that need to be updated.
 
+# It is also possible to use this mechanism to revert prior tags.  In this
+# case, implement a downgrade() method instead.  Dependencies should still
+# work naturally - a tag depending on a tag with a downgrader means that it
+# insists on the other tag being removed and its downgrader executed before
+# its upgrader (or downgrader) can run.  It is still the case that a tag
+# can only be used once.
+
 
 import ConfigParser
 import glob, types, sys, os
@@ -73,6 +80,7 @@ def verboseprint(*args):
 
 class Upgrader:
     tag_set = set()
+    untag_set = set() # tags to remove by downgrading
     by_tag = {}
     legacy = {}
     def __init__(self, filename):
@@ -86,19 +94,26 @@ class Upgrader:
         elif isinstance(self.depends, str):
             self.depends = [self.depends]
 
-        if not hasattr(self, 'upgrader'):
-            print "Error: no upgrader method for", self.tag
-            sys.exit(1)
-        elif not isinstance(self.upgrader, types.FunctionType):
-            print "Error: 'upgrader' for %s is %s, not function", \
-                self.tag, type(self)
+        if hasattr(self, 'upgrader'):
+            if not isinstance(self.upgrader, types.FunctionType):
+                print "Error: 'upgrader' for %s is %s, not function" \
+                    % (self.tag, type(self))
+                sys.exit(1)
+            Upgrader.tag_set.add(self.tag)
+        elif hasattr(self, 'downgrader'):
+            if not isinstance(self.downgrader, types.FunctionType):
+                print "Error: 'downgrader' for %s is %s, not function" \
+                    % (self.tag, type(self))
+                sys.exit(1)
+            Upgrader.untag_set.add(self.tag)
+        else:
+            print "Error: no upgrader or downgrader method for", self.tag
             sys.exit(1)
 
         if hasattr(self, 'legacy_version'):
             Upgrader.legacy[self.legacy_version] = self
 
         Upgrader.by_tag[self.tag] = self
-        Upgrader.tag_set.add(self.tag)
 
     def ready(self, tags):
         for dep in self.depends:
@@ -106,9 +121,15 @@ class Upgrader:
                 return False
         return True
 
-    def upgrade(self, cpt):
-        (self.upgrader)(cpt)
-        verboseprint("applied upgrade for", self.tag)
+    def update(self, cpt, tags):
+        if hasattr(self, 'upgrader'):
+            self.upgrader(cpt)
+            tags.add(self.tag)
+            verboseprint("applied upgrade for", self.tag)
+        else:
+            self.downgrader(cpt)
+            tags.remove(self.tag)
+            verboseprint("applied downgrade for", self.tag)
 
     @staticmethod
     def get(tag):
@@ -173,23 +194,23 @@ def process_file(path, **kwargs):
     # If the current checkpoint has a tag we don't know about, we have
     # a divergence that (in general) must be addressed by (e.g.) merging
     # simulator support for its changes.
-    unknown_tags = tags - Upgrader.tag_set
+    unknown_tags = tags - (Upgrader.tag_set | Upgrader.untag_set)
     if unknown_tags:
         print "warning: upgrade script does not recognize the following "\
               "tags in this checkpoint:", ' '.join(unknown_tags)
 
-    # Apply migrations for tags not in checkpoint, respecting dependences
-    to_apply = Upgrader.tag_set - tags
+    # Apply migrations for tags not in checkpoint and tags present for which
+    # downgraders are present, respecting dependences
+    to_apply = (Upgrader.tag_set - tags) | (Upgrader.untag_set & tags)
     while to_apply:
         ready = set([ t for t in to_apply if Upgrader.get(t).ready(tags) ])
         if not ready:
             print "could not apply these upgrades:", ' '.join(to_apply)
-            print "upgrade dependences impossible to resolve; aborting"
+            print "update dependences impossible to resolve; aborting"
             exit(1)
 
         for tag in ready:
-            Upgrader.get(tag).upgrade(cpt)
-            tags.add(tag)
+            Upgrader.get(tag).update(cpt, tags)
             change = True
 
         to_apply -= ready