[RFC PATCH] xen/design: Add design for EFI dom0less system start

[Luca Fancellu <luca.fancellu@xxxxxxx>]

Add a design describing a proposal to improve the EFI
configuration file, adding keywords to describe domU
guests and allowing to start a dom0less system.

Signed-off-by: Luca Fancellu <luca.fancellu@xxxxxxx>
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 docs/designs/efi-arm-dom0less.md | 105 +++++++++++++++++++++++++++++++
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 create mode 100644 docs/designs/efi-arm-dom0less.md

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+# Xen EFI configuration file
+
+The current configuration file used by Xen when it is started as an EFI
+application is considering only the dom0 guest and doesn't have any
+property to describe and load in memory domU guests.
+Hence currently it's impossible to start a dom0less system using EFI.
+
+# Objective
+
+This document describes the proposed improvement to the Xen EFI
+configuration file to list properly both the dom0 guest and the domU
+guests as well.
+The final goal is to be able to start a dom0less system using EFI.
+
+# Current Xen EFI configuration file
+
+The current configuration file is described by the documentation page
+https://xenbits.xenproject.org/docs/unstable/misc/efi.html.
+
+Here an example:
+
+```
+[global]
+default=section1
+
+[section1]
+options=console=vga,com1 com1=57600 loglvl=all noreboot
+kernel=vmlinuz-3.0.31-0.4-xen [domain 0 command line options]
+ramdisk=initrd-3.0.31-0.4-xen
+xsm=<filename>
+dtb=devtree.dtb
+```
+
+# Proposed improvement
+
+The proposed improvement to the current configuration file is the
+introduction of new keywords to describe additional domUs.
+
+Here follows the proposed new keywords:
+  - domu#_kernel=<kernel file> [domU command line options]
+    - Mandatory kernel file for the domU#
+  - domu#_ramdisk=<ramdisk file>
+    - Optional ramdisk file for the domU#
+  - domu#_dtb=<dtb file>
+    - Optional dtb fragment file for the domU#, it is used for device
+      assignment (passthrough).
+  - domu#_property=cpus=2
+    - Properties that should be added to the dtb in the domU node to
+      properly describe the domU guest. Refer to the documentation:
+      
https://xenbits.xenproject.org/docs/unstable/misc/arm/device-tree/booting.txt,
+      section "Creating Multiple Domains directly from Xen".
+
+For all the keywords above, the # is a number that uniquely identifies
+the guest.
+The keywords domu#_kernel, domu#_ramdisk, domu#_dtb are unique, therefore there
+must not be specified the same keyword twice in a section.
+The # number is not enforcing any domid, it is just used to link each property
+to the right guest, so there can be domu1_* guests that are started with domid 
2
+and so on.
+
+The domu#_property can appear multiple times and it specifies an additional
+property to be listed in the domU node inside the device tree, Xen will
+not check if the same content is specified multiple times.
+
+There are some property whose name starts with an hash symbol (#address-cells,
+#size-cells), in this case the line will be considered as a comment, so to
+specify them, they have to be listed without the hash symbol, the documentation
+will be updated as well to see the implemented handling of these special
+properties.
+
+# Example of a configuration file describing a dom0less system
+
+The following configuration file is describing a dom0less system starting two
+guests.
+
+```
+[global]
+default=xen
+
+[xen]
+# Xen boot arguments
+options=noreboot console=dtuart dtuart=serial0 bootscrub=0
+# Xen device tree
+dtb=devtree.dtb
+
+# Guest 1
+domu1_property=address-cells=2
+domu1_property=size-cells=2
+domu1_kernel=Image-domu1.bin console=ttyAMA0 root=/dev/ram0 rw
+domu1_property=cpus=1
+domu1_property=memory=0xC0000
+domu1_dtb=domu.dtb
+
+# Guest 2
+domu2_kernel=Image-domu2.bin console=ttyAMA0 root=/dev/ram0 rw
+domu2_property=cpus=2
+domu2_property=memory=0x100000
+domu2_property=vpl011
+```
+
+In this example the Guest 1 is loading Image-domu1.bin as kernel and it is 
using
+the passed boot arguments, it is also loading a device tree snippet for device
+passthrough.
+The second guest, Guest 2, is using another kernel and it will use the vpl011
+device.
-- 
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