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Re: [Qemu-devel] [PATCH v7 01/23] docs: update QMP documents for OOB com

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v7 01/23] docs: update QMP documents for OOB commands
Date: Fri, 9 Feb 2018 08:10:53 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2

On 01/23/2018 11:39 PM, Peter Xu wrote:
Update both the developer and spec for the new QMP OOB (Out-Of-Band)

Signed-off-by: Peter Xu <address@hidden>
  docs/devel/qapi-code-gen.txt | 68 ++++++++++++++++++++++++++++++++++++++++----
  docs/interop/qmp-spec.txt    | 30 ++++++++++++++++---
  2 files changed, 89 insertions(+), 9 deletions(-)

diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt
index 06ab699066..4d3db0ad39 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 }

Shouldn't this be '*allow-oob': true, as the only time you add the field is if you turn oob on (as it already defaults to off)?

  Commands are defined by using a dictionary containing several members,
  where three members are most common.  The 'command' member is a
@@ -636,6 +639,59 @@ 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

s/to support/that supports/

+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": { } }

This talks more about the QMP user protocol, while the rest of the document is about QAPI constructs. But we do have an example of 'my-first-command' in the document, so I guess this is okay.

+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 ---

Do we really need the paragraph mentioning a forward reference, when the very next thing is the item that was referenced?

+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

Under normal QMP command execution, the following apply to each command:

+- 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:

When a command is executed with OOB, the following changes occur:

+- They can be executed before an existing command,

They can be completed before a pending in-band command,

+- They run in a monitor dedicated thread,
+- They do not take the BQL during execution.
+OOB command handlers must satisfy the following conditions:
+- It executes extremely fast,
+- It does not take any lock, or, it can take very small locks if all
+  critical regions also follow the rules for OOB command handler code,
+- 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 in doubt, do not implement OOB execution support.
=== Events === @@ -739,10 +795,12 @@ 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.

The "query-qmp-schema" command can be used to inspect which commands support "oob" execution.

+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

s/specific or multiple of/any/

+capabilities, QMP client needs to send "qmp_capabilities" command with

s/QMP/the QMP
s/send/send the/
s/with/with an/

+extra parameter for the capabilities.

s/the/the requested/

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
+- 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: {

To enable "oob" handling of a particular command,

+  "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

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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