path: root/QMP/qmp-spec.txt
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diff --git a/QMP/qmp-spec.txt b/QMP/qmp-spec.txt
index f3c032770..9d30a8ce6 100644
--- a/QMP/qmp-spec.txt
+++ b/QMP/qmp-spec.txt
@@ -215,3 +215,58 @@ Additionally, Clients must not assume any particular:
- Order of json-object members or json-array elements
- Amount of errors generated by a command, that is, new errors can be added
to any existing command in newer versions of the Server
+6. Downstream extension of QMP
+We recommend that downstream consumers of QEMU do *not* modify QMP.
+Management tools should be able to support both upstream and downstream
+versions of QMP without special logic, and downstream extensions are
+inherently at odds with that.
+However, we recognize that it is sometimes impossible for downstreams to
+avoid modifying QMP. Both upstream and downstream need to take care to
+preserve long-term compatibility and interoperability.
+To help with that, QMP reserves JSON object member names beginning with
+'__' (double underscore) for downstream use ("downstream names"). This
+means upstream will never use any downstream names for its commands,
+arguments, errors, asynchronous events, and so forth.
+Any new names downstream wishes to add must begin with '__'. To
+ensure compatibility with other downstreams, it is strongly
+recommended that you prefix your downstram names with '__RFQDN_' where
+RFQDN is a valid, reverse fully qualified domain name which you
+control. For example, a qemu-kvm specific monitor command would be:
+ (qemu) __org.linux-kvm_enable_irqchip
+Downstream must not change the server greeting (section 2.2) other than
+to offer additional capabilities. But see below for why even that is
+Section '5 Compatibility Considerations' applies to downstream as well
+as to upstream, obviously. It follows that downstream must behave
+exactly like upstream for any input not containing members with
+downstream names ("downstream members"), except it may add members
+with downstream names to its output.
+Thus, a client should not be able to distinguish downstream from
+upstream as long as it doesn't send input with downstream members, and
+properly ignores any downstream members in the output it receives.
+Advice on downstream modifications:
+1. Introducing new commands is okay. If you want to extend an existing
+ command, consider introducing a new one with the new behaviour
+ instead.
+2. Introducing new asynchronous messages is okay. If you want to extend
+ an existing message, consider adding a new one instead.
+3. Introducing new errors for use in new commands is okay. Adding new
+ errors to existing commands counts as extension, so 1. applies.
+4. New capabilities are strongly discouraged. Capabilities are for
+ evolving the basic protocol, and multiple diverging basic protocol
+ dialects are most undesirable.