Re: [Qemu-devel] RFC: Monitor high-level design

[Luiz Capitulino]

On Wed, 22 Sep 2010 08:18:10 -0500
Anthony Liguori <address@hidden> wrote:

> On 09/22/2010 07:32 AM, Markus Armbruster wrote:

[...]

> > I can't see why we must have a dogmatic "thou shalt not use anything but
> > QMP to implement HMP commands, ever" rule.

I based most of my ideas on what Anthony has described at:

   http://wiki.qemu.org/Features/QMP_0.14

But that's more like a start point, of course that we shouldn't be dogmatic,
specially when we have better ideas.

> FWIW, print/sum can be easily implemented in terms of QMP commands.

Yes, but I think the point is when/if we should do it.

> It's not really necessary for every possibly HMP command to be 
> implemented in terms of QMP.  However, unless you can do everything in 
> QMP that you can do in HMP, then people will be inclined to use HMP as a 
> management interface.

s/use HMP/use QMP

> So, the only real rule we should try to adopt is that QMP is at least as 
> powerful as HMP.  That should be our goal.

Agreed.

> >>         - HMP can't be moved outside of QEMU, if we want that we'd have to
> >>           write a new Monitor client (potentially in a different language,
> >>           which is actually good)
> >>      
> > I'd rather not worry about that now.
> >
> >    
> >>         - Not sure if HMP features like command completion will perfectly 
> >> fit
> >>      
> > Maybe I'm naive, but I figure that as long as the human monitor controls
> > its character device and knows enough about its own commands, it can do
> > completion pretty much the same way it does now.

Yes, that's right.

> > Let's have a closer look at the human monitor:
> >
> >      +---------------+   reads/writes
> >      | human monitor |<--------------->   character device
> >      +---------------+                          ^
> >              |                                  |
> >              | calls                            |
> >              |                                  |
> >              v                                  |
> >      +---------------+        writes            |
> >      | hmon handlers | -------------------------+
> >      +---------------+
> >
> > The "human monitor" box actually has identifiable sub-boxes:
> >
> > * Command table: human monitor command description
> >
> > * Several argument types: methods to parse, check, complete, ...
> >
> > * Reader: read a line of input from character device
> >
> >    - Readline, uses table for completion
> >
> > * Parser: parse a line of input, uses table to parse arguments
> >
> > * Executor: call the appropriate handler, uses table
> >
> > The machine monitor (short: qmon) is pretty similar, actually.

That's the 'is pretty similar' part I was referring to when I said (by IRC
I guess) that qmon and hmon can share the device handling code.

> > Differences:
> >
> > * We read and parse JSON objects, not lines.
> >
> > * We don't bother to set up completion.
> >
> > * The qmon handlers don't write to the character device.  They return
> >    objects instead, to be written by the qmon executor.
> >
> > Remember that we want qmon handlers to use proper internal interfaces to
> > do the real work.  In other words, qmon handlers should be fairly
> > trivial.
> >
> > I can think of three ways to write a human monitor handler:
> >
> > 1. Call a qmon handler, print the object it returns, suitably formatted.
> >
> > 2. Call the internal interfaces, just like qmon handlers do.  Print the
> >     results.
> >
> >     This cut out the qmon handler middle-man.  Since the qmon handler is
> >     supposed to be trivial, this isn't a big deal.
> >
> > 3. Just do it.  I think that's fine for some commands that make sense
> >     only human monitor, like print.  Also fine for legacy commands
> >     scheduled for removal; we got more useful things to do than
> >     beautifying those.  The ones we want to keep we should convert to
> >     1. or 2.  Doing that early may be advisable to get the internal
> >     interfaces used, but it's not like "need to convert everything before
> >     anything works".
> >
> > You see hmon doesn't use all of qmon, only qmon handlers, and even that
> > only with 1.  Can't see a layering issue there.

Yes and note that we have 1 and 3 today already.

> Yeah, I think what you describe makes perfect sense.  As long as QMP is 
> a thin wrapper around an API, and HMP also calls the API, then it's 
> pretty clear that QMP is at least as powerful as HMP.

Ok, let me get into two details:

 1. The BufferCharDriverState is only going to be used for human monitor
    passthrough, right?

 2. As Markus described above, QMP and HMP perform similar internal steps,
    so what about having them behind a common interface, like this:

    struct monitor_ops {
         const char *name;
         int (*init)(void *opaque);
         void (*exit)(void *opaque);
         int (*can_read)(void *opaque);
         void (*read)(void *opaque, const uint8_t *buf, int size);
         void (*event)(void *opaque, int event);
         void (*async_message)(void *opaque, MonitorEvent, QObject *data);
    };

    Then monitor.c code would register itself with the chardev layer, calling
    each monitor's monitor_ops.

    Note that we already have a feature request to be able to create new 
monitors
    on demand. I believe such design will help.

    Is this overkill? Does anyone have better ideas on how both modules 
(QMP/HMP)
    should be designed/separated?

    Sketches are welcome.