[Qemu-devel] [PATCH v2 2/2] docs: document support lifetime for features

[Daniel P. Berrange]

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 <address@hidden>
---
 qemu-doc.texi | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/qemu-doc.texi b/qemu-doc.texi
index 29f89d8..e3c6400 100644
--- a/qemu-doc.texi
+++ b/qemu-doc.texi
@@ -37,6 +37,7 @@
 * QEMU Guest Agent::
 * QEMU User space emulator::
 * Implementation notes::
+* Support lifetime::
 * Deprecations::
 * License::
 * Index::
@@ -3017,6 +3018,39 @@ Run the emulation in single step mode.
 
 @include qemu-tech.texi
 
address@hidden Support lifetime
address@hidden Support lifetime
+
+Features provided by QEMU are subject to varying support lifetimes.
+
+In general features are intended to be supported indefinitely once
+introduced. In the event that a feature needs to be removed, there
+will be some period in which it will be marked as deprecated before
+eventual removal. All deprecated features will be listed in the
+``Deprecations'' appendix of this document. They may also generate
+warnings on the console when QEMU starts up, or if activated via
+a monitor command.
+
+Certain features will have an inherently finite lifetime, and thus
+will be removed on a fixed schedule.
+
address@hidden Machine types
address@hidden 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 machine types is 12 releases, which is
+equivalent to 4 years worth of previous QEMU releases.
+
 @node Deprecations
 @appendix Deprecations
 
-- 
2.9.3