[Qemu-devel] Re: [RFC] New API for asynchronous monitor commands

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 22 Jan 2010 17:17:27 -0600
Anthony Liguori <anthony@codemonkey.ws> wrote:

> On 01/22/2010 01:03 PM, Adam Litke wrote:
> > Qemu has a number of commands that can operate asynchronously (savevm, migrate,
> > etc) and it will be getting more.  For these commands, the user monitor needs
> > to be suspended, but QMP monitors could continue to to accept other commands.
> > This patch introduces a new command API that isolates the details of handling
> > different monitor types from the actual command execution.
> >
> > A monitor command can use this API by implementing the mhandler.cmd_async
> > handler (or info_async if appropriate).  This function is responsible for
> > submitting the command and does not return any data although it may raise
> > errors.  When the command completes, the QMPCompletion callback should be
> > invoked with its opaque data and the command result.
> >
> > The process for submitting and completing an asynchronous command is different
> > for QMP and user monitors.  A user monitor must be suspended at submit time and
> > resumed at completion time.  The user_print() function must be passed to the
> > QMPCompletion callback so the result can be displayed properly.  QMP monitors
> > are simpler.  No submit time setup is required.  When the command completes,
> > monitor_protocol_emitter() writes the result in JSON format.
> >
> > This API can also be used to implement synchronous commands.  In this case, the
> > cmd_async handler should immediately call the QMPCompletion callback.  It is my
> > hope that this new interface will work for all commands, leading to a
> > drastically simplified monitor.c once all commands are ported.
> >
> > Thanks to Anthony for helping me out with the initial design.
> >
> > Signed-off-by: Adam Litke<agl@us.ibm.com>
> > To: Anthony Liguori<anthony@codemonkey.ws>
> > cc: Luiz Capitulino<lcapitulino@redhat.com>
> > Cc: qemu-devel@nongnu.org
> >    
> 
> I like this a lot and I'd like to see us remove cmd_new in place of 
> cmd_async. The conversion is pretty easy since we just have to add a 
> cb(ret_data) to the end of synchronous functions.
> 
> Luiz/Markus/Avi, what do ya'll think?

 I like it too, but I don't like the idea of using it for synchronous
commands, I explained the reasons in the patch itself.