initrd

README.adoc

@@ -892,7 +892,7 @@ ____
 +
 `-M virt` has some limitations, e.g. I could not pass `-drive if=scsi` as for `arm`, and so <<Snapshot>> fails.
 +
-* uses initramfs. This glues the rootfs as an initrd directly to the kernel image, so the only argument that QEMU needs is the `-kernel`, and no disk! Pretty cool.
+* uses <<initramfs>>, so thre is no filesystem persistency.
 
 So, as long as you keep those points in mind, our `-a aarch64` offers an interesting different setup to play with.
 
@@ -1495,6 +1495,61 @@ Aborted (core dumped)
 
 If we checkout to the ancient kernel `v2.6.22.9`, it fails to compile with modern GNU make 4.1: https://stackoverflow.com/questions/35002691/makefile-make-clean-why-getting-mixed-implicit-and-normal-rules-deprecated-s lol
 
+=== initrd
+
+The kernel can boot from an CPIO file, which is a directory serialization format much like tar: https://superuser.com/questions/343915/tar-vs-cpio-what-is-the-difference
+
+The bootloader, which for us is QEMU itself, is then configured to put that CPIO into memory, and tell the kernel that it is there.
+
+With this setup, you don't even need to give a root filesystem to the kernel, it just does everything in memory in a ramfs.
+
+Try it out with:
+
+....
+./run -i
+....
+
+Notice how it boots fine, even though `-drive` is not given.
+
+Also as expected, there is no filesystem persistency, since we are doing everything in memory:
+
+....
+date >f
+poweroff
+cat f
+# can't open 'f': No such file or directory
+....
+
+The main ingredients to get this working are:
+
+* `BR2_TARGET_ROOTFS_CPIO=y`: make Buildroot generate `output/images/rootfs.cpio` in addition to the other images.
++
+It is also possible to compress that image with other options.
+* `qemu -initrd`: make QEMU put the image into memory and tell the kernel about it.
+* `CONFIG_BLK_DEV_INITRD=y`: Compile the kernel with initrd support
++
+Buildroot forces that option when `BR2_TARGET_ROOTFS_CPIO=y` is given
+
+https://unix.stackexchange.com/questions/89923/how-does-linux-load-the-initrd-image asks how the mechanism works in more detail.
+
+==== initramfs
+
+initramfs is just like initrd, but you also glue the image directly to the kernel image itself.
+
+So the only argument that QEMU needs is the `-kernel`, no `-drive` not even `-initrd`! Pretty cool.
+
+Try it out with:
+
+....
+./run -a aarch64
+....
+
+since our <<aarch64>> setup uses it by default.
+
+In the background, it uses `BR2_TARGET_ROOTFS_INITRAMFS`, and this makes the kernel config option `CONFIG_INITRAMFS_SOURCE` point to the CPIO that will be embedded in the kernel image.
+
+http://nairobi-embedded.org/initramfs_tutorial.html shows a full manual setup.
+
 === ftrace
 
 Trace a single function:

buildroot_config_fragment

@@ -7,6 +7,7 @@ BR2_ROOTFS_OVERLAY="../rootfs_overlay"
 BR2_ROOTFS_POST_BUILD_SCRIPT="../rootfs_post_build_script"
 BR2_ROOTFS_POST_IMAGE_SCRIPT="../rootfs_post_image_script"
 BR2_ROOTFS_USERS_TABLES="../user_table"
+BR2_TARGET_ROOTFS_CPIO=y
 
 # Host GDB
 BR2_GDB_VERSION="7.11.1"

kernel_config_fragment

@@ -1,7 +1,11 @@
 # Changes to this file are automatically trigger kernel reconfigures
 # even without using the linux-reconfigure target.
+#
+# Beware that buildroot can override some of the configurations we make, e.g.
+# it forces CONFIG_BLK_DEV_INITRD=y when BR2_TARGET_ROOTFS_CPIO is on.
 
 CONFIG_MODULE_SRCVERSION_ALL=y
+CONFIG_BLK_DEV_INITRD=y
 
 # GDB debugging.
 CONFIG_DEBUG_FS=y

run

@@ -13,7 +13,9 @@ nographic=false
 #   Turned on by default since v4.12
 extra_append='nokaslr norandmaps printk.devkmsg=on printk.time=y'
 extra_flags=''
-while getopts a:de:knqt:x OPT; do
+initrd=false
+root=''
+while getopts a:de:iknqt:x OPT; do
   case "$OPT" in
     a)
       arch="$OPTARG"
@@ -31,6 +33,9 @@ while getopts a:de:knqt:x OPT; do
       extra_flags="$extra_flags -serial tcp::1234,server,nowait"
       kgdb=true
       ;;
+    i)
+      initrd=true
+      ;;
     n)
       extra_append="$extra_append console=ttyS0"
       extra_flags="$extra_flags -nographic"
@@ -60,13 +65,18 @@ case "$arch" in
     if $kgdb; then
       extra_append="$extra_append kgdboc=ttyS0,115200"
     fi
+    if $initrd; then
+      extra_flags="$extra_flags -initrd '${images_dir}/rootfs.cpio'"
+    else
+      root='root=/dev/vda'
+      extra_flags="$extra_flags -drive file='${images_dir}/rootfs.ext2,if=virtio,format=raw'"
+    fi
     cmd="$qemu_common \
 -M pc \
--append 'root=/dev/vda nopat $extra_append' \
+-append '$root nopat $extra_append' \
--device lkmc_pci_min \
 -device edu \
+-device lkmc_pci_min \
 -device virtio-net-pci,netdev=net0 \
--drive file=${images_dir}/rootfs.ext2.qcow2,if=virtio,format=qcow2 \
 -kernel ${images_dir}/bzImage \
 $extra_flags \
 "
@@ -77,14 +87,16 @@ $extra_flags \
     fi
     cmd="$qemu_common \
 -M versatilepb \
--append 'root=/dev/sda $extra_append' \
+-append '$extra_append' \
 -device rtl8139,netdev=net0 \
--drive file=${images_dir}/rootfs.ext2.qcow2,if=scsi,format=qcow2 \
+-initrd ${images_dir}/rootfs.cpio \
 -dtb ${images_dir}/versatile-pb.dtb \
 -kernel ${images_dir}/zImage \
 -serial stdio \
 $extra_flags \
 "
+#-append 'root=/dev/sda $extra_append' \
+#-drive file=${images_dir}/rootfs.ext2.qcow2,if=scsi,format=qcow2 \
   ;;
   aarch64)
     if $kgdb; then