[Qemu-devel] [RFC v3 ATCH 0/5] char: expose CirMemCharDriver and provide QMP interface

[Lei Li]

This RFC series attempts to convert the MemCharDriver to use a circular
buffer for input and output, expose it to users by introducing QMP commands
memchar_write and memchar_read and via the command line like the other
CharDriverStates.

Serial ports in qemu always use CharDriverStates as there backends,
Right now, all of our backends always try to write the data from the
guest to a socket or file. The concern from OpenStack is that this could
lead to unbounded disk space usage since they log the serial output.
For more detail of the background info:
https://bugs.launchpad.net/nova/+bug/832507

So we want to use a circular buffer in QEMU instead, and then OpenStack
can periodically read the buffer in QEMU and log it.

The QMP commands introduced like:

{ 'command': 'memchar-write',
  'data': {'chardev': 'str', 'size': 'int', 'data': 'str',
           'format': 'str', 'control': 'str' } }

{ 'command': 'memchar-read',
  'data': {'chardev': 'str', 'size': 'int',
           'format': 'str', 'control': 'str' },
  'returns': 'str' }

Expose CirMemCharDriver via the command line like:

qemu -chardev memchr,id=foo,maxcapacity=640k -serial chardev:foo

Introduce HMP command 'console' like:

(qemu) console foo

Note:
Now all of the feature were implemented, this series is just a
sketch and not completely tested. Please comment and let me know
if this seems like the direction we should be headed, your suggestion
would be very appreciated!

Known issues:
There are still some problems I am fixing,
  - The circularMemoryDriver need to be improved.
  - For the 'block' option as sync command, will support it later when
    we gain the necessary infrastructure.
  - The HMP command 'console' still have problems to be fixed. 

Changes since v2:
  - Add congestion mechanism. For the 'block' option as sync command,
    will support it later when we gain the necessary infrastructure
    enhancement.
  - Add HMP 'console' command so that can interact with multiple
    chardevs via a single monitor socket.
  - Make the circular buffer backend and the current MemCharDriver
    live in parallel, expose a new char backend with circular buffer
    CirMemCharDriver suggested by Luiz.
  - Other fixs from Eric and Markus.

Changes since v1:
  - Exposing the MemCharDriver via command line.
  - Support base64 data format suggested by Anthony and Eric.
  - Follow the new rule for the name of qmp command from Eric.


Lei Li (5):
  qemu-char: Add new char device CirMemCharDriver
  Expose CirMemCharDriver via command line
  QAPI: Introduce memchar-write QMP command
  QAPI: Introduce memchar-read QMP command
  HMP: Introduce console command

 hmp-commands.hx  |   72 ++++++++++++++++++
 hmp.c            |   79 ++++++++++++++++++++
 hmp.h            |    3 +
 monitor.c        |   18 +++++
 monitor.h        |    2 +
 qapi-schema.json |   96 ++++++++++++++++++++++++
 qemu-char.c      |  212 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 qemu-config.c    |    3 +
 qemu-options.hx  |   10 +++
 qmp-commands.hx  |   76 +++++++++++++++++++
 10 files changed, 571 insertions(+), 0 deletions(-)