[PATCH RFC] docs: Add minimum version depencency policy document

[George Dunlap <george.dunlap@xxxxxxxxxx>]

Define a specific criteria for how we determine what tools and
libraries to be compatible with.  This will clarify issues such as,
"Should we continue to support Python 2.4" moving forward.

Note that CentOS 7 is set to stop receiving "normal" maintenance
updates in "Q4 2020"; assuming that 4.15 is released after that, we
only need to support CentOS / RHEL 8.

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
---

CC: Ian Jackson <ian.jackson@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Jan Beulich <jbeulich@xxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien@xxxxxxx>
CC: Rich Persaud <persaur@xxxxxxxxx>
CC: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
---
 docs/index.rst                        |  2 +
 docs/policies/dependency-versions.rst | 76 +++++++++++++++++++++++++++
 2 files changed, 78 insertions(+)
 create mode 100644 docs/policies/dependency-versions.rst

diff --git a/docs/index.rst b/docs/index.rst
index b75487a05d..ac175eacc8 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -57,5 +57,7 @@ Miscellanea
 -----------
 
 .. toctree::
+   :maxdepth: 1
 
+   policies/dependency-versions
    glossary
diff --git a/docs/policies/dependency-versions.rst 
b/docs/policies/dependency-versions.rst
new file mode 100644
index 0000000000..d5eeb848d8
--- /dev/null
+++ b/docs/policies/dependency-versions.rst
@@ -0,0 +1,76 @@
+.. SPDX-License-Identifier: CC-BY-4.0
+
+Build and runtime dependencies
+==============================
+
+Xen depends on other programs and libraries to build and to run.
+Chosing a minimum version of these tools to support requires a careful
+balance: Supporting older versions of these tools or libraries means
+that Xen can compile on a wider variety of systems; but means that Xen
+cannot take advantage of features available in newer versions.
+Conversely, requiring newer versions means that Xen can take advantage
+of newer features, but cannot work on as wide a variety of systems.
+
+Specific dependencies and versions for a given Xen release will be
+listed in the toplevel README, and/or specified by the ``configure``
+system.  This document lays out the principles by which those versions
+should be chosen.
+
+The general principle is this:
+
+    Xen should build on currently-supported versions of major distros
+    when released.
+
+"Currently-supported" means whatever that distro's version of "full
+support".  For instance, at the time of writing, CentOS 7 and 8 are
+listed as being given "Full Updates", but CentOS 6 is listed as
+"Maintenance updates"; under this criterium, we would try to ensure
+that Xen could build on CentOS 7 and 8, but not on CentOS 6.
+
+Exceptions for specific distros or tools may be made when appropriate.
+
+One exception to this is compiler versions for the hypervisor.
+Support for new instructions, and in particular support for new safety
+features, may require a newer compiler than many distros support.
+These will be specified in the README.
+
+Distros we consider when deciding minimum versions
+--------------------------------------------------
+
+We currently aim to support Xen building and running on the following 
distributions:
+Debian_,
+Ubuntu_,
+OpenSUSE_,
+Arch Linux,
+SLES_,
+Yocto_,
+CentOS_,
+and RHEL_.
+
+.. _Debian: https://www.debian.org/releases/
+.. _Ubuntu: https://wiki.ubuntu.com/Releases
+.. _OpenSUSE: https://en.opensuse.org/Lifetime
+.. _SLES: https://www.suse.com/lifecycle/
+.. _Yocto: https://wiki.yoctoproject.org/wiki/Releases
+.. _CentOS: https://wiki.centos.org/About/Product
+.. _RHEL: https://access.redhat.com/support/policy/updates/errata
+
+Specific distro versions supported in this release
+--------------------------------------------------
+
+======== ==================
+Distro   Supported releases
+======== ==================
+Debian   10 (Buster)
+Ubuntu   20.10 (Groovy Gorilla), 20.04 (Focal Fossa), 18.04 (Bionic Beaver), 
16.04 (Xenial Xerus)
+OpenSUSE Leap 15.2
+SLES     SLES 11, 12, 15
+Yocto    3.1 (Dunfell)
+CentOS   8
+RHEL     8
+======== ==================
+
+.. note::
+
+   We also support Arch Linux, but as it's a rolling distribution, the
+   concept of "security supported releases" doesn't really apply.
-- 
2.25.1