qemu: use ccache

readme: improve build variants

README.adoc

@@ -9305,6 +9305,7 @@ For example, if you want to keep two builds around, one for the latest Linux ver
 
 ....
 ./build-buildroot
+git -C "$(./getvar linux_src_dir)" fetch --tags --unshallow
 git -C "$(./getvar linux_src_dir)" checkout v4.16
 ./build-buildroot --linux-build-id v4.16
 git -C "$(./getvar linux_src_dir)" checkout -
@@ -9312,7 +9313,9 @@ git -C "$(./getvar linux_src_dir)" checkout -
 ./run --linux-build-id v4.16
 ....
 
-The `-L` option should be passed to all scripts that support it, much like `-a` for the <<cpu-architecture>>, e.g. to step debug:
+The `git fetch --unshallow` is needed the first time because link:configure[] only does a shallow clone of the Linux kernel to save space and time, see also: https://stackoverflow.com/questions/6802145/how-to-convert-a-git-shallow-clone-to-a-full-clone
+
+The `--linux-build-id` option should be passed to all scripts that support it, much like `-a` for the <<cpu-architecture>>, e.g. to step debug:
 
 .....
 ./rungdb --linux-build-id v4.16
@@ -9322,7 +9325,7 @@ This technique is implemented semi-hackishly by moving symlinks around inside th
 
 ==== QEMU build variants
 
-Analogous to the <<linux-kernel-build-variants>> but with the `-Q` option instead:
+Analogous to the <<linux-kernel-build-variants>> but with the `--qemu-build-id` option instead:
 
 ....
 ./build-qemu
@@ -9335,7 +9338,7 @@ git -C "$(./getvar qemu_src_dir)" checkout -
 
 ==== gem5 build variants
 
-Analogous to the <<linux-kernel-build-variants>> but with the `-M` option instead:
+Analogous to the <<linux-kernel-build-variants>> but with the `--gem5-build-id` option instead:
 
 ....
 # Build master.

build-gem5

@@ -69,7 +69,8 @@ else:
         # TODO cross_compile is ignored because the make does not use CC...
         subprocess.check_call(['make', '-C', bootloader64_dir])
         shutil.copy2(os.path.join(bootloader64_dir, 'boot_emm.arm64'), binaries_dir)
-    assert common.run_cmd([
+    assert common.run_cmd(
+        [
             'scons',
             # TODO factor with build.
             '-j', str(multiprocessing.cpu_count()),
@@ -78,7 +79,7 @@ else:
         ] +
         args.extra_scons_args,
         cwd=common.gem5_src_dir,
-        extra_env={'PATH': '/usr/lib/ccache:' + os.environ['PATH']},
+        extra_env={'PATH': common.ccache_dir + ':' + os.environ['PATH']},
     ) == 0
     term_src_dir = os.path.join(common.gem5_src_dir, 'util/term')
     subprocess.check_call(['make', '-C', term_src_dir])

build-qemu

@@ -33,14 +33,14 @@ else:
         args.extra_config_args,
         cwd=common.qemu_build_dir
     )
-    subprocess.check_call(
+    assert common.run_cmd(
         [
             'make',
             # TODO factor with build.
             '-j', str(multiprocessing.cpu_count()),
         ],
         cwd=common.qemu_build_dir,
-        extra_env={'PATH': '/usr/lib/ccache:' + os.environ['PATH']},
-    )
+        extra_env={'PATH': common.ccache_dir + ':' + os.environ['PATH']},
+    ) == 0
     end_time = time.time()
     common.print_time(end_time - start_time)

common.py

@@ -32,6 +32,7 @@ gem5_default_src_dir = os.path.join(submodules_dir, 'gem5')
 linux_src_dir = os.path.join(submodules_dir, 'linux')
 qemu_src_dir = os.path.join(submodules_dir, 'qemu')
 parsec_benchmark_src_dir = os.path.join(submodules_dir, 'parsec-benchmark')
+ccache_dir = os.path.join('/usr', 'lib', 'ccache')
 
 # Other default variables.
 arch_map = {