[Qemu-devel] [RFC PATCH 0/4] char: expose MemoryCharDriver to users 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'} }

{ 'command': 'memchar_read',
  'data': {'chardev': 'str', 'size': 'int'},
  'returns': 'str' }

Expose MemCharDriver via the command line like:

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

Note:
This series is just a incomplete sketch and not completely tested which
I am still struggling with, but I want to get this out ealier to have
your suggestion. Please comment and let me know if this seems like the
direction we should be headed, thanks!

TODO:
1) Expose the MemCharDriver via the command line like the other
   CharDriverStates.
2) Wherther to add locking or notification mechanism.


Lei Li (4):
  qemu-char: Convert MemCharDriver to circular buffer
  monitor: Adjust qmp_human_monitor_command to new MemCharDriver
  QAPI: Introduce memchar_write QMP command
  QAPI: Introduce memchar_read QMP command

 monitor.c        |    2 +-
 qapi-schema.json |   40 +++++++++++++++++
 qemu-char.c      |  130 ++++++++++++++++++++++++++++++++++++++++++++++--------
 qemu-char.h      |    2 +-
 qmp-commands.hx  |   60 ++++++++++++++++++++++++-
 5 files changed, 212 insertions(+), 22 deletions(-)