[Qemu-devel] [PATCH v5 1/2 (for 2.10)] docs: document support lifetime for features

["Daniel P. Berrange" <berrange@redhat.com>]

There is currently no explicit guidance on the duration of support
for features such as versioned machine types, which have a finite
useful lifespan. Thus apps / users cannot predict how much time
they might be able to use a feature for, before it is removed (if
ever).

This adds a new appendix that lists items which have finite lifecycles,
such as machine types. For items which are generally expected to be
supported indefinitely, it sets out the policy around deprecation
and removal, should it be needed.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
---
 qemu-doc.texi | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/qemu-doc.texi b/qemu-doc.texi
index 48af5155c7..200c0f7d0a 100644
--- a/qemu-doc.texi
+++ b/qemu-doc.texi
@@ -38,6 +38,7 @@
 * QEMU Guest Agent::
 * QEMU User space emulator::
 * Implementation notes::
+* Support lifetime::
 * License::
 * Index::
 @end menu
@@ -3128,6 +3129,42 @@ Run the emulation in single step mode.
 
 @include qemu-tech.texi
 
+@node Support lifetime
+@appendix Support lifetime
+
+In general features are intended to be supported indefinitely once
+introduced into QEMU.
+
+In the event that a feature needs to be removed, it will be listed
+in the ``Deprecated features'' appendix of this document. The feature
+will remain functional for 2 releases prior to actual removal.
+
+Deprecated features may also generate warnings on the console when
+QEMU starts up, or if activated via a monitor command, however,
+this is not a mandatory requirement.
+
+Certain features have an inherently finite lifetime, and thus
+will be removed on a fixed schedule, without following the normal
+deprecation process. Such features are listed in the sections
+that follow.
+
+@node Machine types
+@section Machine types
+
+For architectures which aim to support live migration compatibility
+across releases, each release will introduce a new versioned machine
+type. For example, the 2.8.0 release introduced machine types
+``pc-i440fx-2.8'' and ``pc-q35-2.8'' for the x86_64/i686 architectures.
+
+To allow live migration of a guest running on a 2.8.0 release to a
+2.9.0, the QEMU 2.9.0 version must support the ``pc-i440fx-2.8'' and
+``pc-q35-2.8''. To allow users live migrating VMs to skip multiple
+intermediate releases when upgrading, new releases of QEMU will
+support machine types from many previous versions.
+
+The supported lifetime for versioned machine types is 12 releases,
+which is equivalent to 4 years worth of previous QEMU releases.
+
 @node License
 @appendix License
 
-- 
2.13.0