Add steps to compile and boot a custom kernel in RHCOS environment for dev/debug purposes

boot_custom_kernel.md

@@ -0,0 +1,59 @@
+# Steps/Commands to compile and boot into a custom kernel on RHCOS node:
+
+```
+  # cd /var/home/core
+
+  # cat Dockerfile
+  FROM fedora:latest
+
+  RUN dnf install -y kexec-tools make gcc git openssl-devel bison flex autoconf automake bc
+  # 
+
+  *** Setup container image to build the kernel ***
+  # podman build -t fedora:build -f Dockerfile
+
+  # cp /lib/modules/`uname -r`/config .
+
+  # podman run -it -v /var/home/core:/root --privileged=true -- fedora:build bash
+
+  *** Build the kernel in container ***
+  # cd /root
+
+  # git clone --depth=1 git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
+
+  # cp config linux/.config
+
+  # cd linux
+
+  # make oldconfig
+
+  # make vmlinux; make modules
+
+  # make INSTALL_MOD_STRIP=1 INSTALL_MOD_PATH=/root modules_install
+
+  # cp vmlinux /root/vmlinux-<kver>
+
+  # cp /sbin/kexec /root
+
+  # exit
+
+  *** Back to RHCOS host ***
+
+  *** To build initrd image, pick dracut options from existing RHCOS initrd in /boot/ostree and use the same options,
+  *** plus "--kmoddir '/var/home/core/lib/modules/5.9.0-rc5'" to find the modules since we could not install kernel modules
+  *** at the usual place as /lib/modules directory is read-only.  ***
+  # mkdir /tmp/dracut
+  # dracut --reproducible --gzip -v --add 'ostree' --tmpdir '/tmp/dracut' -f --add 'ignition' --no-hostonly --add-drivers 'mptspi vmw_pvscsi' --omit-drivers 'nouveau' --omit 'nfs' --add 'iscsi' --add 'ifcfg' --add 'fips' --kmoddir '/var/home/core/lib/modules/<kver>' initramfs-<kver>.img <kver>
+
+  ** Ready to load and kexec into the just built kernel ***
+  # ./kexec -l --append="`cat /proc/cmdline`" --initrd=initramfs-<kver>.img vmlinux-<kver>
+
+  # ./kexec -e
+
+  Note: After booting past initrd, 'modprobe' would fail to find modules as the modules for the custom kernel could not be placed in
+        /lib/modules directory (read-only) where they are looks for. Three options to workaround it:
+             a. Use alias "modprobe -d /var/home/core" for "modprobe"
+             b. Build the missing kernel modules into the initrd with add-drivers option
+             c. Compile the modules into the kernel (=y instead of =m)
+```
+