Re: Summary of Re: Making QEMU easier for management tools and applications

From: Markus Armbruster <armbru@redhat.com>
To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: "Kevin Wolf" <kwolf@redhat.com>,
	"Peter Maydell" <peter.maydell@linaro.org>,
	"Daniel P. Berrange" <berrange@redhat.com>,
	"Denis V. Lunev" <den@virtuozzo.com>,
	qemu-devel <qemu-devel@nongnu.org>,
	"Marc-André Lureau" <marcandre.lureau@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"John Snow" <jsnow@redhat.com>,
	"Dominik Csapak" <d.csapak@proxmox.com>
Subject: Re: Summary of Re: Making QEMU easier for management tools and applications
Date: Mon, 10 Feb 2020 17:43:13 +0100	[thread overview]
Message-ID: <87ftfio0ke.fsf@dusky.pond.sub.org> (raw)
In-Reply-To: <CAJSP0QWCn_vv2Vs-UWc9nPHjdBQq9KMkiQ7D91+RwHM_okTzZw@mail.gmail.com> (Stefan Hajnoczi's message of "Mon, 10 Feb 2020 10:56:55 +0000")

Stefan Hajnoczi <stefanha@gmail.com> writes:

> On Tue, Feb 4, 2020 at 3:54 PM Markus Armbruster <armbru@redhat.com> wrote:
>> = Ways to provide machine-friendly initial configuration =
>>
>> Two ways to provide machine-friendly initial configuration on par with
>> QMP have been proposed:
>>
>> 1. Extend QMP
>>
>>    Machines use the CLI only to configure a QMP socket.  The remainder
>>    of the CLI becomes human-only, with much relaxed compatibility rules.
>>
>> 2. QAPIfy the CLI
>>
>>    Provide a machine-friendly CLI based on QAPI and JSON.  The current
>>    CLI becomes human-only, with much relaxed compatibility rules.
>
> Do we keep the existing CLI around in both cases?  I'm concerned that
> we're still following the HMP/QMP approach, which has left QEMU with
> the legacy HMP monitor that we still haven't removed.

The "HMP is legacy" idea is relatively recent.

I think having separate interfaces for humans and machines makes sense,
we just need to give both the attention and care they need and deserve.

I think a human-friendly monitor is has its use, but it should ideally
be done differently than we do HMP now.

Likewise, human-friendly initial configuration should exist, but it
should ideally be done differently than we do HMP now.

> I'm in favor of simplifying QEMU at the expense of an incompatible CLI
> change in QEMU 6.0.

I'm convinced the current CLI needs cleanup badly, and that means
incompatible change.  The question is how and when to change it.

Here's how I'd like us to do it:

1. Create machine-friendly initial configuration interface separate from
   the existing CLI

   Doesn't mean it cannot be a CLI.

2. Develop it step by step to feature parity with existing CLI

   If we identify misfeatures we don't want anymore, we should
   immediately deprecate them in the existing CLI instead.

2. Transition machine users to this new interface

3. Declare the existing CLI to be like HMP: for humans, may change
   incompatibly

4. Clean up existing CLI step by step to wrap around the
   machine-friendly interface

   Whatever we deprecated in step 2 goes to the bit bucket instead.

   I'm open to replacing the existing CLI by a separate wrapper process
   instead.

   Capability to translate to the machine-friendly interface is
   desirable, so human users can easily transition to the
   machine-friendly interface when they run into a need to automate.

The risk is of course that we fail at step 4 and remain stuck with the
CLI mess we've made.

> A project like this could prototype incompatible CLI changes in a
> separate git tree.  If it achieves the desired unification (CLI, QMP,
> configuration file) and simplification (less code, legacy removal)
> then it can be merged for an upcoming QEMU major release.

That's effectively a really long-lived feature branch.  Painful.  If it
it what it takes, we do it.  I hope the process I just sketched permits
more incremental development.

We can explore conflict-free in qemu-storage-daemon.