Update both the developer and spec for the new QMP OOB (Out-Of-Band)
command.
Signed-off-by: Peter Xu <pet...@redhat.com>
---
docs/devel/qapi-code-gen.txt | 70 ++++++++++++++++++++++++++++++++++++++++----
docs/interop/qmp-spec.txt | 30 ++++++++++++++++---
2 files changed, 91 insertions(+), 9 deletions(-)
diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt
index f04c63fe82..02cc1a4075 100644
--- a/docs/devel/qapi-code-gen.txt
+++ b/docs/devel/qapi-code-gen.txt
@@ -554,9 +554,12 @@ following example objects:
=== Commands ===
+--- General Command Layout ---
+
Usage: { 'command': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT,
'*returns': TYPE-NAME, '*boxed': true,
- '*gen': false, '*success-response': false }
+ '*gen': false, '*success-response': false,
+ '*allow-oob': false }
Commands are defined by using a dictionary containing several members,
where three members are most common. The 'command' member is a
@@ -636,6 +639,61 @@ possible, the command expression should include the
optional key
'success-response' with boolean value false. So far, only QGA makes
use of this member.
+A command can be declared to support Out-Of-Band (OOB) execution. By
+default, commands do not support OOB. To declare a command to support
+it, we need an extra 'allow-oob' field. For example:
+
+ { 'command': 'migrate_recover',
+ 'data': { 'uri': 'str' }, 'allow-oob': true }
+
+To execute a command in Out-Of-Band way, we need to specify the
+"control" field in the request, with "run-oob" set to true. Example:
+
+ => { "execute": "command-support-oob",
+ "arguments": { ... },
+ "control": { "run-oob": true } }
+ <= { "return": { } }
+
+Without it, even the commands that support out-of-band execution will
+still be run In-Band.
+
+Please read the "Out-Of-Band Command Execution" section below for more
+information on how OOB execution works.
+
+--- About Out-Of-Band (OOB) Command Execution ---
+
+Out-Of-Band does not mean a special kind of command. Instead, it's a
+special way to execute the command. One normal command can be
+declared to support Out-Of-Band execution when 'allow-oob' field is
+set to true when defining the command. With that, it can be run in an
+Out-Of-Band way if 'run-oob' is specified in 'control' field of
+command request.
+
+When we say normal QMP command executions, it means basically the
+following:
+
+- They are executed in order,
+- They run only in main thread of QEMU,
+- They have the BQL taken during execution.
+
+For OOB command executions, they differ in the following:
+
+- They can be executed before an existing command,
+- They run in a monitor dedicated thread,
+- They do not take the BQL during execution.
+
+To add OOB execution support for a command, we need to make sure the
+command handler satisfies at least the following:
+
+- It executes extremely fast,
+- It does not take any lock (or, it can take very small locks, but in
+ very predictable ways),
+- It does not invoke system calls that may block,
+- It does not access guest RAM that may block when userfaultfd is
+ enabled for postcopy live migration.
+
+If someone is unsure about whether a command handler can be run in OOB
+way, then it possibly means that it does not suite for OOB execution.
=== Events ===
@@ -739,10 +797,12 @@ references by name.
QAPI schema definitions not reachable that way are omitted.
The SchemaInfo for a command has meta-type "command", and variant
-members "arg-type" and "ret-type". On the wire, the "arguments"
-member of a client's "execute" command must conform to the object type
-named by "arg-type". The "return" member that the server passes in a
-success response conforms to the type named by "ret-type".
+members "arg-type", "ret-type" and "allow-oob". On the wire, the
+"arguments" member of a client's "execute" command must conform to the
+object type named by "arg-type". The "return" member that the server
+passes in a success response conforms to the type named by
+"ret-type". When "allow-oob" is set, it means the command supports
+out-of-band execution.
If the command takes no arguments, "arg-type" names an object type
without members. Likewise, if the command returns nothing, "ret-type"
diff --git a/docs/interop/qmp-spec.txt b/docs/interop/qmp-spec.txt
index f8b5356015..e20163c138 100644
--- a/docs/interop/qmp-spec.txt
+++ b/docs/interop/qmp-spec.txt
@@ -83,16 +83,27 @@ The greeting message format is:
2.2.1 Capabilities
------------------
-As of the date this document was last revised, no server or client
-capability strings have been defined.
+Currently supported capabilities are:
+- "oob": it means the QMP server supports "Out-Of-Band" command
+ execution. For more detail, please see "run-oob" parameter in
+ "Issuing Commands" section below. Not all commands allow this "oob"
+ execution. One can know whether one command supports "oob" by
+ "query-qmp-schema" command.
+
+QMP clients can get a list of supported QMP capabilities of the QMP
+server in the greeting message mentioned above. By default, all the
+capabilities are off. To enable a specific or multiple of QMP
+capabilities, QMP client needs to send "qmp_capabilities" command with
+extra parameter for the capabilities.
2.3 Issuing Commands
--------------------
The format for command execution is:
-{ "execute": json-string, "arguments": json-object, "id": json-value }
+{ "execute": json-string, "arguments": json-object, "id": json-value,
+ "control": json-dict }
Where,
@@ -102,10 +113,16 @@ The format for command execution is:
required. Each command documents what contents will be considered
valid when handling the json-argument
- The "id" member is a transaction identification associated with the
- command execution, it is optional and will be part of the response if
+ command execution. It is required if OOB is enabled, and optional
+ if not. The same "id" field will be part of the response if
provided. The "id" member can be any json-value, although most
clients merely use a json-number incremented for each successive
command
+- The "control" member is optional, and currently only used for
+ "out-of-band" execution ("oob" as shortcut). The handling or
+ response of an "oob" command can overtake prior in-band commands.
+ To enable "oob" feature, just provide a control field with: {
+ "control": { "run-oob": true } }
2.4 Commands Responses
----------------------
@@ -113,6 +130,11 @@ The format for command execution is:
There are two possible responses which the Server will issue as the result
of a command execution: success or error.
+As long as the commands were issued with a proper "id" field, then the
+same "id" field will be attached in the corresponding response message
+so that requests and responses can match. Clients should drop all the
+responses that are with unknown "id" field.
+
2.4.1 success
-------------
--
2.14.3
