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[5/6] docs: Deprecate CPU model runnability guarantees

Message ID 20190625050008.12789-6-ehabkost@redhat.com
State New
Headers show
Series x86 CPU model versioning | expand

Commit Message

Eduardo Habkost June 25, 2019, 5 a.m. UTC 
Document that CPU model runnability guarantees won't apply to
unversioned CPU models anymore.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
Cc: libvir-list@redhat.com
---
 qemu-deprecated.texi | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)
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Patch

diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
index df04f2840b..5fafb62a6d 100644
--- a/qemu-deprecated.texi
+++ b/qemu-deprecated.texi
@@ -243,3 +243,22 @@  Note that if you are exposing the export via /dev/nbd0, it is easier
 to just export the entire image and then mount only /dev/nbd0p1 than
 it is to reinvoke @command{qemu-nbd -c /dev/nbd0} limited to just a
 subset of the image.
+
+@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.