Re: [Qemu-devel] [PATCH v4 00/20] monitor: add asynchronous command type

["Marc-André Lureau" <marcandre.lureau@gmail.com>]

On Tue, Apr 9, 2019 at 6:12 PM Marc-André Lureau
<marcandre.lureau@redhat.com> wrote:
>
> Hi,
>
> HMP and QMP commands are handled synchronously in qemu today. But
> there are benefits allowing the command handler to re-enter the main
> loop if the command cannot be handled synchronously, or if it is
> long-lasting. Some bugs such as rhbz#1230527 are difficult to solve
> without it.
>
> The common solution is to use a pair of command+event in this case.
> But this approach has a number of issues:
> - you can't "fix" an existing command: you need a new API, and ad-hoc
>   documentation for that command+signal association, and old/broken
>   command deprecation
> - since the reply event is broadcasted and 'id' is used for matching the
>   request, it may conflict with other clients request 'id' space
> - it is arguably less efficient and elegant (weird API, useless return
>   in most cases, broadcast reply, no cancelling on disconnect etc)
>
> The following series implements an async command solution instead. By
> introducing a session context and a command return handler, it can:
> - defer the return, allowing the mainloop to reenter
> - return only to the caller (instead of broadcast events for reply)
> - optionnally allow cancellation when the client is gone
> - track on-going qapi command(s) per client/session
>
> and without introduction of new QMP APIs or client visible change.
>
> Existing qemu commands can be gradually replaced by async:true
> variants when needed, while carefully reviewing the concurrency
> aspects. The async:true commands marshaller helpers are splitted in
> half, the calling and return functions. The command is called with a
> QmpReturn context, that can return immediately or later, using the
> generated return helper, which allows for a step-by-step conversion.
>
> The screendump command is converted to an async:true version to solve
> rhbz#1230527. The command shows basic cancellation (this could be
> extended if needed). It could be further improved to do asynchronous
> IO writes as well.
>
> v4:
> - rebased, mostly adapting to new OOB code
>   (there was not much feedback in v3 for the async command part,
>    but preliminary patches got merged!)
> - drop the RFC status

ping

>
> v3:
> - complete rework, dropping the asynchronous commands visibility from
>   the protocol side entirely (until there is a real need for it)
> - rebased, with a few preliminary cleanup patches
> - teach asynchronous commands to HMP
>
> v2:
> - documentation fixes and improvements
> - fix calling async commands sync without id
> - fix bad hmp monitor assert
> - add a few extra asserts
> - add async with no-id failure and screendump test
>
> Marc-André Lureau (20):
>   qmp: constify QmpCommand and list
>   json-lexer: make it safe to call destroy multiple times
>   qmp: add QmpSession
>   QmpSession: add a return callback
>   QmpSession: add json parser and use it in qga
>   monitor: use qmp session to parse json feed
>   qga: simplify dispatch_return_cb
>   QmpSession: introduce QmpReturn
>   qmp: simplify qmp_return_error()
>   QmpSession: keep a queue of pending commands
>   QmpSession: return orderly
>   qmp: introduce asynchronous command type
>   scripts: learn 'async' qapi commands
>   qmp: add qmp_return_is_cancelled()
>   monitor: add qmp_return_get_monitor()
>   console: add graphic_hw_update_done()
>   console: make screendump asynchronous
>   monitor: start making qmp_human_monitor_command() asynchronous
>   monitor: teach HMP about asynchronous commands
>   hmp: call the asynchronous QMP screendump to fix outdated/glitches
>
>  qapi/misc.json                          |   3 +-
>  qapi/ui.json                            |   3 +-
>  scripts/qapi/commands.py                | 151 ++++++++++++++---
>  scripts/qapi/common.py                  |  15 +-
>  scripts/qapi/doc.py                     |   3 +-
>  scripts/qapi/introspect.py              |   3 +-
>  hmp.h                                   |   3 +-
>  include/monitor/monitor.h               |   3 +
>  include/qapi/qmp/dispatch.h             |  89 +++++++++-
>  include/qapi/qmp/json-parser.h          |   7 +-
>  include/ui/console.h                    |   5 +
>  hmp.c                                   |   6 +-
>  hw/display/qxl-render.c                 |   9 +-
>  hw/display/qxl.c                        |   1 +
>  monitor.c                               | 198 ++++++++++++++--------
>  qapi/qmp-dispatch.c                     | 214 +++++++++++++++++++-----
>  qapi/qmp-registry.c                     |  33 +++-
>  qga/commands.c                          |   2 +-
>  qga/main.c                              |  51 ++----
>  qobject/json-lexer.c                    |   5 +-
>  qobject/json-streamer.c                 |   3 +-
>  tests/test-qmp-cmds.c                   | 206 +++++++++++++++++++----
>  ui/console.c                            | 100 +++++++++--
>  hmp-commands.hx                         |   3 +-
>  tests/qapi-schema/qapi-schema-test.json |   5 +
>  tests/qapi-schema/qapi-schema-test.out  |   8 +
>  tests/qapi-schema/test-qapi.py          |   8 +-
>  27 files changed, 877 insertions(+), 260 deletions(-)
>
> --
> 2.21.0.196.g041f5ea1cf
>
>


-- 
Marc-André Lureau