Doc: Improve the kdump sysconfig document

Currently, the kdump sysconfig document is missed, let's add it to
the kexec-kdump-howto.txt as a document in order to help users better
understand these options in the /etc/sysconfig/kdump.

Signed-off-by: Lianbo Jiang <lijiang@redhat.com>
Acked-by: Kairui Song <kasong@redhat.com>

kexec-kdump-howto.txt

@@ -524,17 +524,91 @@ to send over the necessary ssh key file. Restart the kdump service via
 Advanced Setups
 ===============
 
-Kdump boot directory
+About /etc/sysconfig/kdump
---------------------
+------------------------------
+
+Currently, there are a few options in /etc/sysconfig/kdump, which are
+usually used to control the behavior of kdump kernel. Basically, all of
+these options have default values, usually we do not need to change them,
+but sometimes, we may modify them in order to better control the behavior
+of kdump kernel such as debug, etc.
+
+-KDUMP_BOOTDIR
 
 Usually kdump kernel is the same as 1st kernel. So kdump will try to find
 kdump kernel under /boot according to /proc/cmdline. E.g we execute below
 command and get an output:
 	cat /proc/cmdline
 	BOOT_IMAGE=/xxx/vmlinuz-3.yyy.zzz  root=xxxx .....
-Then kdump kernel will be /boot/xxx/vmlinuz-3.yyy.zzz.
+
-However a variable KDUMP_BOOTDIR in /etc/sysconfig/kdump is provided to
+Then kdump kernel will be /boot/xxx/vmlinuz-3.yyy.zzz. However, this option
-user if kdump kernel is put in a different directory.
+is provided to user if kdump kernel is put in a different directory.
+
+-KDUMP_IMG
+
+This represents the image type used for kdump. The default value is "vmlinuz".
+
+-KDUMP_IMG_EXT
+
+This represents the images extension. Relocatable kernels don't have one.
+Currently, it is a null string by default.
+
+-KEXEC_ARGS
+
+Any additional kexec arguments required. For example:
+KEXEC_ARGS="--elf32-core-headers".
+
+In most situations, this should be left empty. But, sometimes we hope to get
+additional kexec loading debugging information, we can add the '-d' option
+for the debugging.
+
+-KDUMP_KERNELVER
+
+This is a kernel version string for the kdump kernel. If the version is not
+specified, the init script will try to find a kdump kernel with the same
+version number as the running kernel.
+
+-KDUMP_COMMANDLINE
+
+The value of 'KDUMP_COMMANDLINE' will be passed to kdump kernel as command
+line parameters, this will likely match the contents of the grub kernel line.
+
+In general, if a command line is not specified, which means that it is a null
+string such as KDUMP_COMMANDLINE="", the default will be taken automatically
+from the '/proc/cmdline'.
+
+-KDUMP_COMMANDLINE_REMOVE
+
+This option allows us to remove arguments from the current kdump command line.
+If we don't specify any parameters for the KDUMP_COMMANDLINE, it will inherit
+all values from the '/proc/cmdline', which is not expected. As you know, some
+default kernel parameters could affect kdump, furthermore, that could cause
+the failure of kdump kernel boot.
+
+In addition, the option is also helpful to debug the kdump kernel, we can use
+this option to change kdump kernel command line.
+
+For more kernel parameters, please refer to kernel document.
+
+-KDUMP_COMMANDLINE_APPEND
+
+This option allows us to append arguments to the current kdump command line
+after processed by the KDUMP_COMMANDLINE_REMOVE. For kdump kernel, some
+specific modules require to be disabled like the mce, cgroup, numa, hest_disable,
+etc. Those modules may waste memory or kdump kernel doesn't need them,
+furthermore, there may affect kdump kernel boot.
+
+Just like above option, it can be used to disable or enable some kernel
+modules so that we can exclude any errors for kdump kernel, this is very
+meaningful for debugging.
+
+-KDUMP_STDLOGLVL | KDUMP_SYSLOGLVL | KDUMP_KMSGLOGLVL
+
+These variables are used to control the kdump log level in the first kernel.
+In the second kernel, kdump will use the rd.kdumploglvl option to set the log
+level in the above KDUMP_COMMANDLINE_APPEND.
+
+Logging levels: no logging(0), error(1), warn(2), info(3), debug(4)
 
 Kdump Post-Capture Executable
 -----------------------------