gem5: update to 4c8efdbef45d98109769cf675ee3411393e8ed06

The Linux boot was still broken after the previous gem5 update. Let's try
a new one...

Add the --no-ccache option to benchmark builds.

README.adoc

@@ -20574,6 +20574,8 @@ Get results with:
 tail -n+1 ../linux-kernel-module-cheat-regression/*/gem5-bench-build-*.txt
 ....
 
+<<p51>>, Ubuntu 19.10, LKMC 7c6bb29bc89ec3f1056c0680c3f08bd64018a7bc GCC 9.2.1, gem5 d7d9bc240615625141cd6feddbadd392457e49eb (18-02-2020), `./build --arch aarch64 --gem5-worktree master --no-cache`: 19:33 TODO must investigate why it got so much worse.
+
 ====== Benchmark gem5 single file change rebuild time
 
 This is the critical development parameter, and is dominated by the link time of huge binaries.
@@ -21155,6 +21157,14 @@ We have https://buildroot.org/downloads/manual/manual.html#ccache[enabled ccache
 * absolute paths are used and GDB can find source files
 * but builds are not reused across separated LKMC directories
 
+ccache can be disabled with the `--no-ccache` option as in:
+
+....
+./build-gem5 --no-ccache
+....
+
+This can be useful to <<benchmark-build,benchmark builds>>.
+
 === getvar
 
 The link:getvar[] helper script can print the values of internal LKMC variables.

build-gem5

@@ -121,6 +121,9 @@ https://github.com/cirosantilli/linux-kernel-module-cheat-regression#gem5-unit-t
                 gold_linker_cmd = []
             else:
                 gold_linker_cmd = ['--gold-linker', LF]
+        kwargs = {}
+        if self.env['ccache']:
+            kwargs['extra_paths'] = [self.env['ccache_dir']]
         exit_status = self.sh.run_cmd(
             (
                 [
@@ -141,9 +144,9 @@ https://github.com/cirosantilli/linux-kernel-module-cheat-regression#gem5-unit-t
                 self.sh.add_newlines(self.env['extra_scons_args'])
             ),
             cwd=self.env['gem5_source_dir'],
-            extra_paths=[self.env['ccache_dir']],
             extra_env=extra_env,
             raise_on_failure = False,
+            **kwargs
         )
         return exit_status
 

common.py

@@ -221,6 +221,13 @@ CPU architecture to use. If given multiple times, run the action
 for each arch sequentially in that order. If one of them fails, stop running.
 Valid archs: {}
 '''.format(arches_string)
+        )
+        self.add_argument(
+            '--ccache',
+            default=True,
+            help='''\
+Enable or disable ccache: https://cirosantilli.com/linux-kernel-module-cheat#ccache
+'''
         )
         self.add_argument(
             '--dry-run',