investigate squashfs to overcome BR2_TARGET_ROOTFS_EXT2_SIZE but fail

Generate qcow2 when running QEMU if it is not present.

README.adoc

@@ -222,7 +222,7 @@ We use `./build-buildroot` because the kernel modules go inside the root filesys
 
 The reboot after rebuild is annoying. We don't have a perfect solution for it yet, but there are some ideas cooking at: <<gem5-restore-new-script>>.
 
-Using <<KVM>> can speed boot up however if your host and guest have the same arch, e.g. on an `x86_64` host:
+Using <<KVM>> can <<benchmark-linux-kernel-boot,speed up>> the boot however if your host and guest have the same arch, e.g. on an `x86_64` host:
 
 ....
 ./run --kvm
@@ -7103,6 +7103,7 @@ But TODO we didn't get it working yet:
 
 * https://stackoverflow.com/questions/41119656/how-can-i-overlayfs-the-root-filesystem-on-linux
 * https://unix.stackexchange.com/questions/316018/how-to-use-overlayfs-to-protect-the-root-filesystem
+* https://unix.stackexchange.com/questions/420646/mount-root-as-overlayfs
 
 Test with the script:
 
@@ -7129,11 +7130,13 @@ ls /overlay/proc
 
 A less good alternative is to set `LD_LIBRARY_PATH` on the 9p mount and run executables directly from the mount.
 
-Even mor awesome than `chroot` be to `pivot_root`, but I couldn't get that working either:
+Even more awesome than `chroot` would be to `pivot_root`, but I couldn't get that working either:
 
 * https://stackoverflow.com/questions/28015688/pivot-root-device-or-resource-busy
 * https://unix.stackexchange.com/questions/179788/pivot-root-device-or-resource-busy
 
+Here is a more basic working example of OverlayFS usage: https://askubuntu.com/questions/109413/how-do-i-use-overlayfs/1075564#1075564
+
 === Guest host networking
 
 First ensure that networking is enabled before trying out anything in this section: <<networking>>
@@ -9298,11 +9301,22 @@ Unfortunately, TODO we don't have a perfect way to find the right value for `BR2
 du -hsx "$(./getvar --arch arm target_dir)"
 ....
 
-https://stackoverflow.com/questions/49211241/is-there-a-way-to-automatically-detect-the-minimum-required-br2-target-rootfs-ex
+Some promising ways to overcome this problem include:
 
-libguestfs is very promising link:https://serverfault.com/questions/246835/convert-directory-to-qemu-kvm-virtual-disk-image/916697#916697[], in particular link:http://libguestfs.org/guestfish.1.html#vfs-minimum-size[`vfs-minimum-size`].
+* <<squashfs>>
+TODO benchmark: would gem5 suffer a considerable disk read performance hit due to decompressing SquashFS?
+* libguestfs: link:https://serverfault.com/questions/246835/convert-directory-to-qemu-kvm-virtual-disk-image/916697#916697[], in particular link:http://libguestfs.org/guestfish.1.html#vfs-minimum-size[`vfs-minimum-size`]
+* use methods described at: <<gem5-restore-new-script>> instead of putting builds on the root filesystem
 
-One way to overcome this problem is to mount benchmarks from host instead of adding them to the root filesystem, e.g. with: <<9p>>.
+Bibliography: https://stackoverflow.com/questions/49211241/is-there-a-way-to-automatically-detect-the-minimum-required-br2-target-rootfs-ex
+
+==== SquashFS
+
+link:https://en.wikipedia.org/wiki/SquashFS[SquashFS] creation with `mksquashfs` does not take fixed sizes, and I have successfully booted from it, but it is readonly, which is unacceptable.
+
+But then we could mount link:https://wiki.debian.org/ramfs[ramfs] on top of it with <<overlayfs>> to make it writable, but my attempts failed exactly as mentioned at <<overlayfs>>.
+
+This is the exact unanswered question: https://unix.stackexchange.com/questions/343484/mounting-squashfs-image-with-read-write-overlay-for-rootfs
 
 [[rpath]]
 === Buildroot rebuild is slow when the root filesystem is large

build-buildroot

@@ -97,7 +97,7 @@ https://stackoverflow.com/questions/49260466/why-when-i-change-br2-linux-kernel-
 That target builds the root filesystem and all its dependencies.'''
     )
     kernel_module_group.add_argument(
-        '--no-kernel-modules', default=defaults['kernel_modules'], action='store_true',
+        '--no-kernel-modules', default=defaults['no_kernel_modules'], action='store_true',
         help="Don't build the kernel modules package"
     )
     parser.add_argument(
@@ -293,18 +293,11 @@ def main(args, extra_args=None):
             cwd=common.buildroot_src_dir,
         ) == 0
 
-        # Create the qcow2 from ext2. This is optional, because gem5
-        # does not need the qcow2.
-        if os.path.exists(common.qemu_img_executable) and os.path.exists(common.ext2_file):
-            assert common.run_cmd([
-                common.qemu_img_executable,
-                '-T', 'pr_manager_run,file=/dev/null',
-                'convert',
-                '-f', 'raw',
-                '-O', 'qcow2',
-                common.ext2_file,
-                common.qcow2_file,
-            ]) == 0
+        # Create the qcow2 from ext2.
+        # Skip if qemu is not present, because gem5 does not need the qcow2.
+        # so we don't force a QEMU build for gem5.
+        if not args.no_all and os.path.exists(common.qemu_img_executable):
+            common.raw_to_qcow2()
 
         return 0
 

buildroot_config/default

@@ -16,8 +16,11 @@ BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
 # Rootfs
 BR2_TARGET_ROOTFS_CPIO=n
 BR2_TARGET_ROOTFS_EXT2=y
-BR2_TARGET_ROOTFS_INITRAMFS=n
 BR2_TARGET_ROOTFS_EXT2_SIZE="512M"
+BR2_TARGET_ROOTFS_SQUASHFS=n
+BR2_TARGET_ROOTFS_INITRAMFS=n
+# TODO can you boot with those as root filesystem?
+BR2_TARGET_ROOTFS_TAR=n
 
 # Host GDB
 BR2_GDB_VERSION="7.11.1"

common.py

@@ -219,6 +219,18 @@ def print_time(ellapsed_seconds):
     minutes, seconds = divmod(rem, 60)
     print("time {:02}:{:02}:{:02}".format(int(hours), int(minutes), int(seconds)))
 
+def raw_to_qcow2():
+    global this
+    assert this.run_cmd([
+        this.qemu_img_executable,
+        '-T', 'pr_manager_run,file=/dev/null',
+        'convert',
+        '-f', 'raw',
+        '-O', 'qcow2',
+        this.rootfs_raw_file,
+        this.qcow2_file,
+    ]) == 0
+
 def resolve_args(defaults, args, extra_args):
     if extra_args is None:
         extra_args = {}
@@ -331,8 +343,8 @@ def setup(parser, **extra_args):
     this.host_dir = os.path.join(this.buildroot_build_dir, 'host')
     this.host_bin_dir = os.path.join(this.host_dir, 'usr', 'bin')
     this.images_dir = os.path.join(this.buildroot_build_dir, 'images')
-    this.ext2_file = os.path.join(this.images_dir, 'rootfs.ext2')
-    this.qcow2_file = os.path.join(this.images_dir, 'rootfs.ext2.qcow2')
+    this.rootfs_raw_file = os.path.join(this.images_dir, 'rootfs.squashfs')
+    this.qcow2_file = this.rootfs_raw_file + '.qcow2'
     this.staging_dir = os.path.join(this.buildroot_build_dir, 'staging')
     this.target_dir = os.path.join(this.buildroot_build_dir, 'target')
     this.run_dir_base = os.path.join(this.out_dir, 'run')

kernel_config/default

@@ -1,11 +1,14 @@
 # Misc.
 CONFIG_BLK_DEV_INITRD=y
-CONFIG_DEBUG_FS=y
 CONFIG_DYNAMIC_DEBUG=y
 CONFIG_MODULE_SRCVERSION_ALL=y
-CONFIG_OVERLAY_FS=y
 CONFIG_STRICT_DEVMEM=n
 
+# Filesystems.
+CONFIG_DEBUG_FS=y
+CONFIG_OVERLAY_FS=y
+CONFIG_SQUASHFS=y
+
 # GDB debugging.
 CONFIG_DEBUG_INFO=y
 CONFIG_DEBUG_KERNEL=y

run

@@ -125,7 +125,7 @@ def main(args, extra_args=None):
                 os.path.join(common.gem5_src_dir, 'configs', 'example', 'arm', 'fs_bigLITTLE.py'),
                 '--big-cpus', '2',
                 '--cpu-type', 'atomic',
-                '--disk', common.ext2_file,
+                '--disk', common.rootfs_raw_file,
                 '--dtb', os.path.join(common.gem5_system_dir, 'arm', 'dt', 'armv8_gem5_v1_big_little_2_2.dtb'),
                 '--kernel', common.vmlinux,
                 '--little-cpus', '2'
@@ -138,7 +138,7 @@ def main(args, extra_args=None):
                 extra_emulator_args.extend(['-r', str(sorted(cpt_dirs).index(cpt_dir) + 1)])
             cmd += [
                 common.gem5_fs_file,
-                '--disk-image', common.ext2_file,
+                '--disk-image', common.rootfs_raw_file,
                 '--kernel', common.vmlinux,
                 '--mem-size', memory,
                 '--num-cpus', str(args.cpus),
@@ -159,6 +159,19 @@ def main(args, extra_args=None):
     else:
         extra_emulator_args.extend(extra_qemu_args)
         os.makedirs(common.run_dir, exist_ok=True)
+        if args.prebuilt:
+            common.mkdir()
+            qemu_executable = "qemu-system-{}".format(args.arch)
+        else:
+            qemu_executable = common.qemu_executable
+        if not os.path.exists(qemu_executable):
+            raise Exception('QEMU executable does not exist, did you forget to build or install it?\n' \
+                            'Tried to use: ' + qemu_executable)
+        if not os.path.exists(common.qcow2_file):
+            if not os.path.exists(common.rootfs_raw_file):
+                raise Exception('Root filesystem not found. Did you build it?\n' \
+                                'Tried to use: ' + common.rootfs_raw_file)
+            common.raw_to_qcow2()
         if args.debug_vm:
             serial_monitor = []
         else:
@@ -167,11 +180,6 @@ def main(args, extra_args=None):
             extra_emulator_args.append('-enable-kvm')
         if args.kgdb:
             extra_emulator_args.extend(['-serial', 'tcp::{},server,nowait'.format(common.gdb_port)])
-        if args.prebuilt:
-            common.mkdir()
-            qemu_executable = "qemu-system-{}".format(args.arch)
-        else:
-            qemu_executable = common.qemu_executable
         cmd = (
             debug_vm +
             [
@@ -212,12 +220,6 @@ def main(args, extra_args=None):
                 '-drive',
                 'file={},format=qcow2,if={}{}{}'.format(common.qcow2_file, driveif, snapshot, rrid)
             ])
-            if not os.path.exists(common.qcow2_file):
-                raise Exception(
-                    'Cannot find the qcow2 root filesystem. You must build QEMU\n'
-                    'before building the root filesystem? That is needed because the qcow2\n' +
-                    'is created with qemu-img. Tried to use: ' + qemu_executable
-                )
             if rr:
                 extra_emulator_args.extend([
                 '-drive', 'driver=blkreplay,if=none,image=img-direct,id=img-blkreplay',
@@ -257,9 +259,6 @@ def main(args, extra_args=None):
                 ] +
                 virtio_gpu_pci
             )
-        if not os.path.exists(qemu_executable):
-            raise Exception('QEMU executable does not exist, did you forget to build or install it?\n' +
-                            'Tried to use: ' + qemu_executable)
     if args.tmux:
         if args.gem5:
             subprocess.Popen([os.path.join(common.root_dir, 'tmu'),