gem5: --gem5-src

.gitignore

@@ -15,3 +15,8 @@ gitignore*
 # Python trash.
 *.pyc
 __pycache__
+
+# Accidents.
+/m5out
+*.o
+*.out

README.adoc

@@ -10657,6 +10657,47 @@ git -C data/gem5/some-branch checkout some-branch-v2
 
 `--gem5-worktree` is only required if you have multiple gem5 checkouts, e.g. it would not be required for multiple builds of the same tree, e.g. a <<gem5-debug-build>> and a non-debug one.
 
+===== gem5 private source trees
+
+Suppose that you are working on a private fork of gem5, but you want to use this repository to develop it as well.
+
+Simply adding your private repository as a remote to `submodules/gem5` is dangerous, as you might forget and push your private work by mistake one day.
+
+Even removing remotes is not safe enough, since `git submodule update` and other submodule commands can restore the old public remote.
+
+Instead, we provide the following safer process.
+
+First do a separate private clone of you private repository:
+
+....
+git clone https://my.private.repo.com/my-fork/gem5.git gem5-internal
+gem5_internal="$(pwd)/gem5-internal"
+....
+
+Next, when you want to build with this repository, use the `--gem5-src` argument to point this repository to the private source code:
+
+....
+cd linux-kernel-module-cheat
+./build-gem5 \
+  --gem5-build-id p/lkmc/master \
+  --gem5-src "$gem5_internal" \
+  --gem5-worktree p/lkmc/master \
+;
+./run-gem5
+  --gem5-build-id p/lkmc/master \
+  --gem5-src "$gem5_internal" \
+  --gem5-worktree p/lkmc/master \
+;
+....
+
+`p/lkmc/` is not mandatory but it provides a sane default:
+
+* `p` stands for `private`, and will separate the private from public builds
+* `lkmc` is added to prevent name conflicts with other work-trees of the private repository
+* `master`, finally, reflects that the worktree is at the `master` state of private repository
+
+This setup only creates gitignored worktrees of the private repository inside this repository, which is pretty safe, while still allowing fully use our infrastructure as usual.
+
 ===== gem5 debug build
 
 The `gem5.debug` has optimizations turned off unlike the default `gem5.opt`, and provides a much better <<debug-the-emulator,debug experience>>:

common.py

@@ -31,7 +31,6 @@ kernel_modules_src_dir = os.path.join(this.packages_dir, 'kernel_modules')
 submodules_dir = os.path.join(root_dir, 'submodules')
 buildroot_src_dir = os.path.join(submodules_dir, 'buildroot')
 crosstool_ng_src_dir = os.path.join(submodules_dir, 'crosstool-ng')
-gem5_default_src_dir = os.path.join(submodules_dir, 'gem5')
 linux_src_dir = os.path.join(submodules_dir, 'linux')
 extract_vmlinux = os.path.join(linux_src_dir, 'scripts', 'extract-vmlinux')
 qemu_src_dir = os.path.join(submodules_dir, 'qemu')
@@ -126,6 +125,10 @@ See the documentation for other values known to work.
         '-M', '--gem5-build-id', default=default_build_id,
         help='gem5 build ID. Allows you to keep multiple separate gem5 builds. Default: %(default)s'
     )
+    parser.add_argument(
+        '--gem5-src',
+        help='If given, use the gem5 directory exactly as submodules/gem5 is would normally used.'
+    )
     parser.add_argument(
         '-N', '--gem5-worktree',
         help='''\
@@ -512,6 +515,10 @@ def setup(parser):
     this.crosstool_ng_executable = os.path.join(this.crosstool_ng_util_dir, 'ct-ng')
     this.crosstool_ng_build_dir = os.path.join(this.crosstool_ng_buildid_dir, 'build')
     this.crosstool_ng_download_dir = os.path.join(this.crosstool_ng_out_dir, 'download')
+    if (args.gem5_src):
+        this.gem5_default_src_dir = args.gem5_src
+    else:
+        this.gem5_default_src_dir = os.path.join(submodules_dir, 'gem5')
     if args.gem5_worktree is not None:
         this.gem5_src_dir = os.path.join(this.gem5_non_default_src_root_dir, args.gem5_worktree)
     else: