Re: [RFC PATCH 0/9] Initial support for machine creation via QMP

[Markus Armbruster <armbru@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> Hi Mirela, this is very interesting!
>
> It's unfortunate that I completely missed the discussions in
> January/February.  You might have noticed that in the 5.2/6.0
> timeframe I worked on cleaning up the machine initialization phases
> and qemu_init.   The idea behind the cleanup was to identify clearly
> the steps from one phase to the next.  I am very happy that you are
> already benefitting from that work in this series and you were able to
> make a prototype with so little code.
>
> My plan was a bit more ambitious though :) and it is laid out at
> https://wiki.qemu.org/User:Paolo_Bonzini/Machine_init_sequence.
>
> My plan was to have completely different binaries than the current
> qemu-system-*.

Now you have 2x problems :)

>                 These would only have a handful of command line
> options (such as -name, -sandbox, -trace, -L) and would open a QMP
> connection on stdio.
>
> All other command line option would be either considered legacy or
> adjusted to be part of two new QMP commands, machine-set and
> accel-set, which would advance through the phases like this:
>
> PHASE_NO_MACHINE
>    -> machine-set -> PHASE_MACHINE_CREATED ->
>    -> accel-set -> PHASE_ACCEL_CREATED -> PHASE_MACHINE_INITIALIZED ->
>    -> finish-machine-init -> PHASE_MACHINE_READY
>    -> cont

Is machine-set one big command, or a sequence of commands, where each
command configures just one thing?

Same for accel-set.

Permit me to go off on a tangent: how much and what kind of magic do we
want in the initialization sequence?

The existing order is a confusing mess grown out of a half-hearted
attempt to make things just work.  We've talked about it a few times,
e.g. here:

    Message-ID: <87poomg77m.fsf@dusky.pond.sub.org>
    https://lists.nongnu.org/archive/html/qemu-devel/2016-09/msg00163.html

Are you aiming for "leave ordering to the user", or for "do the right
thing" (the user's order doesn't matter)", or for "neither; confusing
messes are fun"?

Another tangent:

1. A command line option is like a QMP command that returns nothing.
Generating CLI options from a QAPI schema is no harder than generating
QMP commands, it's just work.

2. A configuration file is like a command line, only easier to work with
when configuration becomes non-trivial.  Generating configuration file
language from a QAPI schema is no harder than generating QMP commands /
CLI options, it's just work.

3. QMP is strictly more powerful than CLI or comfiguration file.  It's
also less convenient for ad hoc use by humans: compare -readconfig
vm123.cfg to feeding the equivalent QMP commands with socat.

4. We'll see whether the convenience for humans can motivate us to put
in the work.

> I think this idea would work well with your plan below, because the
> preconfiguration that you need to do happens mostly at 
> PHASE_MACHINE_INITIALIZED.
>
> Of course, the disadvantage of my approach is that, in the initial
> version, a lot of capabilities of QEMU are not available (especially 
> with respect to the UI, since there's no "display-add"
> command). However, the basic implementation of machine-set and
> accel-set should not really be *that* much more work, and it should be
> doable in parallel with the conversion efforts for those options.  For
> example, today I posted a series that maps -smp to -M (and then, SMP
> configuration would automatically become available in machine-set).
>
> I have placed the skeleton of the work I was doing at
> https://gitlab.com/bonzini/qemu/ in the branch qemu-qmp-targets.  You 
> can see a sample execution at
> https://asciinema.org/a/TXMX8EZh8Md0fZnuE7uhfv6cO.  I have not touched 
> some of the patches in a long time, but I plan to give them a quick
> test tomorrow.  Starting from the code in that branch, it should not
> be hard to implement the machine-set and accel-set commands in the
> same fashion as QEMU 5.2's implementation of object-add.
>
> Thanks for posting these patches, I have started a light review of them.

Please cc: on future work in this area.

I'm about to drop off for two weeks of vacation, though :)