docs: Deprecate CPU model runnability guarantees

Document that CPU model runnability guarantees won't apply to
unversioned CPU models anymore.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20190628002844.24894-9-ehabkost@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
index e6891c3a93..c90b08d553 100644
--- a/qemu-deprecated.texi
+++ b/qemu-deprecated.texi
@@ -292,3 +292,22 @@ subset of the image.
 In the future, QEMU will require Python 3 to be available at
 build time.  Support for Python 2 in scripts shipped with QEMU
 is deprecated.
+
+@section Backwards compatibility
+
+@subsection Runnability guarantee of CPU models (since 4.1.0)
+
+Previous versions of QEMU never changed existing CPU models in
+ways that introduced additional host software or hardware
+requirements to the VM.  This allowed management software to
+safely change the machine type of an existing VM without
+introducing new requirements ("runnability guarantee").  This
+prevented CPU models from being updated to include CPU
+vulnerability mitigations, leaving guests vulnerable in the
+default configuration.
+
+The CPU model runnability guarantee won't apply anymore to
+existing CPU models.  Management software that needs runnability
+guarantees must resolve the CPU model aliases using te
+``alias-of'' field returned by the ``query-cpu-definitions'' QMP
+command.