Luiz Capitulino<address@hidden> writes:
Hi there,
I was working on a detailed writeup about monitor's internals so that I could
get some guidance regarding monitor's internal design, but after today's call
I realized that we should discuss the general design first.
I think we have two options: the first (and better known) is to make HMP
(the human monitor) a QMP client. The other option would be to make QMP and
HMP monitor implementations.
Below I try to introduce both ideas, showing advantages and potential problems.
I've also tried drawing some nice diagrams. Please, be polite and appreciate
them whether you agree or not :-)
1. HMP as a QMP client
Very briefly, QMP would export a function called qmp_command(), which would
be called by HMP handlers. This has been proposed by Anthony and a
detailed description can be found at:
http://wiki.qemu.org/Features/QMP_0.14#Potential_Solutions
When fully implemented, I think it would look like this:
|-----|
| HMP |
|-----|
/ \
/ \
|-----| \
| QMP | \
|-----| \
| \
| \
|---------| |---------|
| chardev | | chardev |
|---------| |---------|
HMP will need to handle its own chardev, so that it's able to output
data to the user (and I guess command completion needs it too).
However, it's important to notice that HMP won't be using QMP's chardev
in any way. It's only there to show that QMP and HMP will handle their
own chardevs.
Advantages:
- QMP's interface is also used (and thus tested) internally
- In theory HMP can be moved outside of QEMU
Disadvantages/problems:
- What to do with handlers that make no sense in QMP, like print,
sum, etc?
I can't see why we must have a dogmatic "thou shalt not use anything but
QMP to implement HMP commands, ever" rule.
- 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.
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.
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.
Now consider human command pass-through. The qmon handler for the
pass-through command needs to pass a command string to a human monitor,
and capture its output to the character device. Thus, it needs to use
hmon parser and executor, with a special character device to capture
output.
There is no dependency cycle, because only the pass-through handler
depends on (parts of) hmon, and while hmon may depend on some qmon
handlers, the pass-through handler is not among them.