XT-distro
=========

XT-distro (XT stands for Xen-Troops) is a set of tools designed to automate
build process of various components of a hypervisor based system, e.g. allows to
build multiple images which are used to run guest OSes. The distribution allows
inter image dependencies, e.g. you can use artifacts coming from one image in
the other one. In particular, while working with Xen hypervisor [1], this allows
easy deployment of Linux kernel images coming from image builds other then Dom0.

XT-distro local.conf extensions
===============================

In order to configure inter-image relations and provide means for other control
there are number of options one can use in local.conf file for better tuning.

XT_SHARED_ROOTFS_DIR – shared directory path used by all the builds to deliver
artifacts. This is in particular, used to deploy Linux Kernel images from
DomU(s) to the known location, so Dom0 can pick those while building its root
file system.

XT_SSTATE_CACHE_MIRROR_DIR – directory path used to deliver SSTATE cache of
builds run by XT-distro. If set, this folder will be used to deliver “partial”
state caches of all the builds, so those caches can be re-used. For example, if
two domains/images are built with XT-distro, e.g. Dom0 and DomU, then after the
successful build state caches of Dom0 and DomU will be deployed to the mirror
directory.

FIXME: we cannot use XT_SSTATE_CACHE_MIRROR_DIR for all the builds directly
because of the fact that some older versions of Poky/OE cannot properly handle
buildhistroy and sstate_cache: history is built during do_fetch, but when
sstate_cache is in use fetch is not called.

XT_ALLOW_SSTATE_CACHE_MIRROR_USE - set to "1" to enable SSTATE_CACHE_MIRROR
usage for inner builds: this will set SSTATE_CACHE_MIRROR in corresponding
local.conf file(s).
N.B. This must not be used if build history is needs to be provided.

XT_POPULATE_SSTATE_CACHE - if set then every domain build will be requested to
populate its build cache (sstate, ccache), e.g. if there are two images are
built (Dom0 and DomU), then both will be asked to populate corresponding caches.

XT_POPULATE_SDK – if set then every domain build will be requested to populate
SDK, e.g. if there are two images are built (Dom0 and DomU), then both will be
asked to populate corresponding SDKs.

XT-distro and bitbake related extensions
========================================

In order to better control inner Yocto based builds there are number of
extensions defined in recipe files.

XT_BB_LAYERS_FILE – a file within inner build source directory (${S})  to be
used as bblayers.conf file.

XT_BB_LOCAL_CONF_FILE - a file within inner build source directory (${S})  to be
used as local.conf file.

XT_BB_IMAGE_TARGET – inner build's bitbake target to be invoked for the build.

XT_QUIRK_BB_ADD_LAYER – list of bitbake layers that need to be added to the
inner build. This is useful if using layers which are not in the inner build's
bblayers.conf file.

XT_BB_CONFIG_CMD/XT_BB_RUN_CMD - most of the time the Yocto init script is
poky/oe-init-build-env, but there are cases (AGL or any other distro other
than poky) when this is not true. Allow recipes override defaults if needed.
These variables should set a command to be used while configuring distro
and setting environment after its configuration done, e.g. resuming
build with bitbake on another shell.

XT-distro based Xen-troops build system
=======================================

The aim of the Xen-troops build system is to automate build of different system
components, such as domain images, into a single delivery. In order to achive
this Xen-troops uses the following additional components:

1. meta-xt-products - collection of manifests with additional components
(meta layers) which in conjunction with xt-distro allow to build a product.
This normally includes xt-distro, meta-xt-images and one of the
meta-xt-prod-XXX:

meta-xt-products/
├── prod-devel.xml
├── prod-XXX.xml

2. meta-xt-images - collection of recipes which allow to build various images
for different domains regardless of the product being built:

meta-xt-images/
├── classes
├── conf
├── recipes-dom0
│   └── dom0-image-weston
│       └── dom0-image-weston.bb
└── recipes-domu
    └── domu-image-weston
        └── domu-image-weston.bb

3. meta-xt-prod-devel - collection of recipes/bbappend files which are used to
tailor generic image recipes to the needs of the product being built

meta-xt-prod-devel/
├── conf
├── recipes-dom0
│   └── dom0-image-weston
│       ├── dom0-image-weston.bbappend
│       └── files
│           └── meta-xt-extra
│               ├── conf
│               │   └── layer.conf
│               └── recipes-domu
│                   └── kernel
│                       ├── core-image-weston.bbappend
│                       └── dom0-image-weston-domu-copy-kernel.bb
├── recipes-domu
│   └── domu-image-weston
│       ├── domu-image-weston.bbappend
│       └── files
│           └── meta-xt-extra
│               ├── conf
│               │   └── layer.conf
│               └── recipes-kernel
│                   └── linux
│                       ├── linux-kernel.inc
│                       ├── linux-renesas_%.bbappend
│                       └── linux-yocto_%.bbappend
└── recipes-xt-image
    └── xt-image
        └── xt-image.bb

Normally a product will have additional meta layer(s) deployed into "inner"
(image) build system: these become a part of that build system
("files/meta-xt-extra" in the example above, used to extend Yocto based build
system for dom0 and domu images).
Additional layers/recipes may exist to populate configuration files or run
configuration scripts or tailor the image in any other way.

NOTE: There is a requirement for each product to have a default target
"xt-image" so it can be invoked by the build scripts.

4. build-scripts - set of scripts which automate product build

[1] https://www.xenproject.org/