================================
Early creation of mapped devices
================================

It is possible to configure a device-mapper device to act as the root device for
your system in two ways.

The first is to build an initial ramdisk which boots to a minimal userspace
which configures the device, then pivot_root(8) in to it.

The second is to create one or more device-mappers using the module parameter
"dm-mod.create=" through the kernel boot command line argument.

The format is specified as a string of data separated by commas and optionally
semi-colons, where:

 - a comma is used to separate fields like name, uuid, flags and table
   (specifies one device)
 - a semi-colon is used to separate devices.

So the format will look like this::

 dm-mod.create=<name>,<uuid>,<minor>,<flags>,<table>[,<table>+][;<name>,<uuid>,<minor>,<flags>,<table>[,<table>+]+]

Where::

	<name>		::= The device name.
	<uuid>		::= xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx | ""
	<minor>		::= The device minor number | ""
	<flags>		::= "ro" | "rw"
	<table>		::= <start_sector> <num_sectors> <target_type> <target_args>
	<target_type>	::= "verity" | "linear" | ... (see list below)

The dm line should be equivalent to the one used by the dmsetup tool with the
`--concise` argument.

Target types
============

Not all target types are available as there are serious risks in allowing
activation of certain DM targets without first using userspace tools to check
the validity of associated metadata.

======================= =======================================================
`cache`			constrained, userspace should verify cache device
`crypt`			allowed
`delay`			allowed
`era`			constrained, userspace should verify metadata device
`flakey`		constrained, meant for test
`linear`		allowed
`log-writes`		constrained, userspace should verify metadata device
`mirror`		constrained, userspace should verify main/mirror device
`raid`			constrained, userspace should verify metadata device
`snapshot`		constrained, userspace should verify src/dst device
`snapshot-origin`	allowed
`snapshot-merge`	constrained, userspace should verify src/dst device
`striped`		allowed
`switch`		constrained, userspace should verify dev path
`thin`			constrained, requires dm target message from userspace
`thin-pool`		constrained, requires dm target message from userspace
`verity`		allowed
`writecache`		constrained, userspace should verify cache device
`zero`			constrained, not meant for rootfs
======================= =======================================================

If the target is not listed above, it is constrained by default (not tested).

Examples
========
An example of booting to a linear array made up of user-mode linux block
devices::

  dm-mod.create="lroot,,,rw, 0 4096 linear 98:16 0, 4096 4096 linear 98:32 0" root=/dev/dm-0

This will boot to a rw dm-linear target of 8192 sectors split across two block
devices identified by their major:minor numbers.  After boot, udev will rename
this target to /dev/mapper/lroot (depending on the rules). No uuid was assigned.

An example of multiple device-mappers, with the dm-mod.create="..." contents
is shown here split on multiple lines for readability::

  dm-linear,,1,rw,
    0 32768 linear 8:1 0,
    32768 1024000 linear 8:2 0;
  dm-verity,,3,ro,
    0 1638400 verity 1 /dev/sdc1 /dev/sdc2 4096 4096 204800 1 sha256
    ac87db56303c9c1da433d7209b5a6ef3e4779df141200cbd7c157dcb8dd89c42
    5ebfe87f7df3235b80a117ebc4078e44f55045487ad4a96581d1adb564615b51

Other examples (per target):

"crypt"::

  dm-crypt,,8,ro,
    0 1048576 crypt aes-xts-plain64
    babebabebabebabebabebabebabebabebabebabebabebabebabebabebabebabe 0
    /dev/sda 0 1 allow_discards

"delay"::

  dm-delay,,4,ro,0 409600 delay /dev/sda1 0 500

"linear"::

  dm-linear,,,rw,
    0 32768 linear /dev/sda1 0,
    32768 1024000 linear /dev/sda2 0,
    1056768 204800 linear /dev/sda3 0,
    1261568 512000 linear /dev/sda4 0

"snapshot-origin"::

  dm-snap-orig,,4,ro,0 409600 snapshot-origin 8:2

"striped"::

  dm-striped,,4,ro,0 1638400 striped 4 4096
  /dev/sda1 0 /dev/sda2 0 /dev/sda3 0 /dev/sda4 0

"verity"::

  dm-verity,,4,ro,
    0 1638400 verity 1 8:1 8:2 4096 4096 204800 1 sha256
    fb1a5a0f00deb908d8b53cb270858975e76cf64105d412ce764225d53b8f3cfd
    51934789604d1b92399c52e7cb149d1b3a1b74bbbcb103b2a0aaacbed5c08584
