Dom0 kernel and ramdisk modules
================================

Xen is passed the dom0 kernel and initrd via a reference in the /chosen
node of the device tree.

Each node contains the following properties:

- compatible

	Must always include at least the generic compatiblity string:

		"multiboot,module"

	Optionally a more specific compatible string may be used in
	addition to the above. One of:

	- "multiboot,kernel"	-- the dom0 kernel
	- "multiboot,ramdisk"	-- the dom0 ramdisk
	- "xen,xsm-policy"	-- XSM policy blob

	It is normally recommended to include a more specific
	compatible string (if one applies) in addition to the generic
	string (which must always be present).

	Xen will assume that the first module which lacks a more
	specific compatible string is a "multiboot,kernel" and that
	the second such is a "multiboot,ramdisk". Any subsequent
	modules which lack a specific compatiblity string will not
	receive any special treatment.

	Xen 4.4 supported a different set of legacy compatible strings
	which remain supported such that systems supporting both 4.4
	and later can use a single DTB.

	- "xen,multiboot-module" equivalent to "multiboot,module"
	- "xen,linux-zimage"     equivalent to "multiboot,kernel"
	- "xen,linux-initrd"     equivalent to "multiboot,ramdisk"

	For compatibility with Xen 4.4 the more specific "xen,linux-*"
	names are non-optional and must be included.

- reg

	Specifies the physical address of the module in RAM and the
	length of the module.

- bootargs (optional)

	Command line associated with this module. See below for the
	priority of this field vs. other mechanisms of specifying the
	bootargs for the kernel.

Examples
========

A boot module of unspecified type:

	module@0xc0000000 {
		compatible = "multiboot,module";
		reg = <0xc0000000 0x1234>;
		bootargs = "...";
	};

A boot module containing a ramdisk:

	module@0xd0000000 {
		compatible = "multiboot,ramdisk", "multiboot,module";
		reg = <0xd0000000 0x5678>;
	};

The previous examples are compatible with Xen 4.5+ only.

To be compatible with Xen 4.4 as well use the legacy names:

	module@0xd0000000 {
		compatible = "xen,linux-initrd", "xen,multiboot-module";
		reg = <0xd0000000 0x5678>;
	};

Command lines
=============

Xen also checks for properties directly under /chosen to find suitable command
lines for Xen and Dom0. The logic is the following:

 - If xen,xen-bootargs is present, it will be used for Xen.
 - If xen,dom0-bootargs is present, it will be used for Dom0.
 - If xen,xen-bootargs is _not_ present, but xen,dom0-bootargs is,
   bootargs will be used for Xen.
 - If a kernel boot module is present and has a bootargs property then
   the top-level bootargs will used for Xen.
 - If no Xen specific properties are present, bootargs is for Dom0.
 - If xen,xen-bootargs is present, but xen,dom0-bootargs is missing,
   bootargs will be used for Dom0.

Most of these cases is to make booting with Xen-unaware bootloaders easier.
For those you would hardcode the Xen commandline in the DTB under
/chosen/xen,xen-bootargs and would let the bootloader set the Dom0 command
line by writing bootargs (as for native Linux).
A Xen-aware bootloader would set xen,xen-bootargs for Xen, xen,dom0-bootargs
for Dom0 and bootargs for native Linux.