Re: [PATCH v4] Documentation: bootconfig: Update boot configuration documentation

Randy Dunlap
2020-03-04T04:55:42Z

Hi again, :)

On 3/3/20 1:05 AM, Masami Hiramatsu wrote:
> Update boot configuration documentation.
>
> - Not using "config" abbreviation but configuration or description.
> - Rewrite descriptions of node and its maxinum number.
> - Add a section of use cases of boot configuration.
> - Move how to use bootconfig to earlier section.
> - Fix some typos, indents and format mistakes.
>
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> ---
> Changes in v4:
> - Remove O= option from examples.
> Changes in v3:
> - Specify that comments also count in size.
> - Fix a confusing sentence.
> - Add O=<builddir> to make command.
> Changes in v2:
> - Fixes additional typos (Thanks Markus and Randy!)
> - Change a section title to "Tree Structured Key".
> ---
> Documentation/admin-guide/bootconfig.rst | 181 +++++++++++++++++++-----------
> Documentation/trace/boottime-trace.rst | 2
> 2 files changed, 117 insertions(+), 66 deletions(-)
>
> diff --git a/Documentation/admin-guide/bootconfig.rst b/Documentation/admin-guide/bootconfig.rst
> index cf2edcd09183..b719b257b579 100644
> --- a/Documentation/admin-guide/bootconfig.rst
> +++ b/Documentation/admin-guide/bootconfig.rst
> @@ -11,19 +11,99 @@ Boot Configuration
> Overview
> ========
>
> -The boot configuration expands the current kernel command line to support
> -additional key-value data when booting the kernel in an efficient way.
> -This allows administrators to pass a structured-Key config file.
> +Boot configuration expands the current kernel command line to support
> +additional key-value data while booting the kernel in an efficient way.
> +This allows administrators to pass a structured key configuration file
> +as a way to supplement the kernel command line to pass system boot parameters.
>
> -Config File Syntax
> -==================
> +Compared with the kernel command line, the boot configuration can provide
> +scalability (up to 32 KiB configuration data including comments), readability
> +(structured configuration with comments) and compact expression of option
> +groups.
> +
> +When to Use the Boot Configuration?
> +-----------------------------------
> +
> +The boot configuration supports kernel command line options and init daemon
> +boot options. All sub-keys under "kernel" root key are passed as a part of
> +kernel command line [1]_, and ones under "init" root key are passed as a part
> +of init command line. For example, ::
> +
> + root=UUID=8cd79b08-bda0-4b9d-954c-5d5f34b98c82 ro quiet splash console=ttyS0,115200n8 console=tty0
> +
> +This can be written as following boot configuration file.::
> +
> + kernel {
> + root = "UUID=8cd79b08-bda0-4b9d-954c-5d5f34b98c82" # nvme0n1p3
> + ro # mount rootfs as read only
> + quiet # No console log
> + splash # show splash image on boot screen
> + console = "ttyS0,115200n8" # 1st console to serial device
> + console += tty0 # add 2nd console
> + }
> +
> +If you think that kernel/init options becomes too long to write in boot-loader
> +configuration file or you want to comment on each option, the boot
> +configuration may be suitable. If unsure, you can still continue to use the
> +legacy kernel command line.
> +
> +Also, some features may depend on the boot configuration, and it has own

and each such
feature has its own root key.

> +root key. For example, ftrace boot-time tracer uses "ftrace" root key to
> +describe its options [2]_. If you want to use such features, you need to
> +enable the boot configuration.
> +
> +.. [1] See :ref:`Documentation/admin-guide/kernel-parameters.rst <kernelparameters>`
> +.. [2] See :ref:`Documentation/trace/boottime-trace.rst <boottimetrace>`
> +
> +
> +How to Use the Boot Configuration?
> +----------------------------------
> +
> +To enable the boot configuration support on your kernel, it must be built with
> +``CONFIG_BOOT_CONFIG=y`` and ``CONFIG_BLK_DEV_INITRD=y``.
> +
> +Next, you can write a boot configuration file and attach it to initrd image.
> +
> +The boot configuration file is attached to the end of the initrd (initramfs)
> +image file with size, checksum and 12-byte magic word as below.
> +
> +[initrd][bootconfig][size(u32)][checksum(u32)][#BOOTCONFIG\n]
> +
> +The Linux kernel decodes the last part of the initrd image in memory to
> +get the boot configuration data.
> +Because of this "piggyback" method, there is no need to change or
> +update the boot loader and the kernel image itself.

boot loader or the kernel image itself.

> +
> +To do this operation, Linux kernel provides "bootconfig" command under

provides the "bootconfig" command under

> +tools/bootconfig, which allows admin to apply or delete the configuration
> +file to/from initrd image. You can build it by the following command::

to/from an initrd image.

> +
> + # make -C tools/bootconfig
> +
> +To add your boot configuration file to initrd image, run bootconfig as below

to an initrd image,

> +(Old data is removed automatically if exists)::

if it exists)::

> +
> + # tools/bootconfig/bootconfig -a your-config /boot/initrd.img-X.Y.Z
>
> -The boot config syntax is a simple structured key-value. Each key consists
> -of dot-connected-words, and key and value are connected by ``=``. The value
> -has to be terminated by semi-colon (``;``) or newline (``\n``).
> +To remove the configuration from the image, you can use -d option as below::

you can use the -d option as below::

> +
> + # tools/bootconfig/bootconfig -d /boot/initrd.img-X.Y.Z
> +
> +At last, add ``bootconfig`` on the normal kernel command line to tell the
> +kernel to look for the bootconfig at the end of the initrd file. For example::
> +
> + GRUB_CMDLINE_LINUX="bootconfig"
> +
> +
> +Boot Configuration Syntax
> +=========================
> +
> +The boot configuration syntax is a simple structured key-value. Each key
> +consists of dot-connected-words, and key and value are connected by ``=``.
> +The value has to be terminated by semi-colon (``;``) or newline (``\n``).
> For array value, array entries are separated by comma (``,``). ::

values,
or just
For an array, its entries are separated by

>
> -KEY[.WORD[...]] = VALUE[, VALUE2[...]][;]
> + KEY[.WORD[...]] = VALUE[, VALUE2[...]][;]
>
> Unlike the kernel command line syntax, spaces are OK around the comma and ``=``.
>
> @@ -39,11 +119,11 @@ you can not escape these quotes.
> There can be a key which doesn't have value or has an empty value. Those keys
> are used for checking if the key exists or not (like a boolean).
>
> -Key-Value Syntax
> -----------------
> +Tree Structured Key
> +-------------------
>
> -The boot config file syntax allows user to merge partially same word keys
> -by brace. For example::
> +The boot configuration syntax allows user to merge same parent key using

allows the user
although I am having problems with the rest of that sentence.

> +braces as tree structured key. For example::
>
> foo.bar.baz = value1
> foo.bar.qux.quux = value2
> @@ -80,19 +160,17 @@ you can use ``+=`` operator. For example::
> In this case, the key ``foo`` has ``bar``, ``baz`` and ``qux``.
>
> However, a sub-key and a value can not co-exist under a parent key.
> -For example, following config is NOT allowed.::
> +For example, following configuration is NOT allowed.::

example, the following

>
> foo = value1
> - foo.bar = value2 # !ERROR! subkey "bar" and value "value1" can NOT co-exist
> + foo.bar = value2 # !ERROR! sub-key "bar" and value "value1" can NOT co-exist
>
>
> Comments
> --------
>
> -The config syntax accepts shell-script style comments. The comments starting
> -with hash ("#") until newline ("\n") will be ignored.
> -
> -::
> +The boot configuration accepts shell-script style comments. The comments
> +starting with hash (``#``) until newline (``\n``), will be skipped.::

no comma. or 2 commas:

The comments,
beginning with hash (``#``) and continuing until newline (``\n``), will be skipped.::

>
> # comment line
> foo = value # value is set to foo.
> @@ -100,74 +178,45 @@ with hash ("#") until newline ("\n") will be ignored.
> 2, # 2nd element
> 3 # 3rd element
>
> -This is parsed as below::
> +This is parsed as below.::
>
> foo = value
> bar = 1, 2, 3
>
> Note that you can not put a comment between value and delimiter(``,`` or
> -``;``). This means following config has a syntax error ::
> +``;``). This means following description has a syntax error. ::

This means the following

>
> - key = 1 # comment
> + key = 1 # !ERROR! comment is not allowed before delimiter
> ,2
>
>
> /proc/bootconfig
> ================
>

[snip]

> Config File Limitation
> ======================
>
> -Currently the maximum config size size is 32KB and the total key-words (not
> -key-value entries) must be under 1024 nodes.
> -Note: this is not the number of entries but nodes, an entry must consume
> -more than 2 nodes (a key-word and a value). So theoretically, it will be
> -up to 512 key-value pairs. If keys contains 3 words in average, it can
> -contain 256 key-value pairs. In most cases, the number of config items
> -will be under 100 entries and smaller than 8KB, so it would be enough.
> +Currently the maximum configuration file size (including comments) is 32 KiB
> +and the total number of key-words and values must be under 1024 nodes.
> +(Note: Each key consists of words separated by dot, and value also consists
> +of values separated by comma. Here, each word and each value is generally
> +called a "node".)

[blank line would be nice here]

> +Theoretically, it will be up to 512 key-value pairs. If keys contains 3
> +words in average, it can contain 256 key-value pairs. In most cases,
> +the number of configuration items will be under 100 entries and smaller
> +than 8 KiB, so it would be enough.
> If the node number exceeds 1024, parser returns an error even if the file

the parser

> -size is smaller than 32KB.
> -Anyway, since bootconfig command verifies it when appending a boot config
> -to initrd image, user can notice it before boot.
> +size is smaller than 32 KiB.
> +Anyway, since bootconfig command verifies it when appending a boot

since the bootconfig command

> +configuration to initrd image, user need to fix it before boot.

to an initrd image, the user needs to fix any errors before boot.

--
~Randy