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Re: qmp-shell for GSoC/Outreachy?

From: Kevin Wolf
Subject: Re: qmp-shell for GSoC/Outreachy?
Date: Thu, 6 Feb 2020 15:21:43 +0100
User-agent: Mutt/1.12.1 (2019-06-15)

Am 06.02.2020 um 10:40 hat Markus Armbruster geschrieben:
> >> On 2/5/20 8:09 AM, Kevin Wolf wrote:
> >> > Am 28.01.2020 um 11:59 hat Kevin Wolf geschrieben:
> >> >>>> The other part that it needs to solve is how to be available by 
> >> >>>> default
> >> >>>> without specifying anything on the command line. Basically, if I press
> >> >>>> Ctrl-Alt-2, I want to get to a monitor shell. If that shell is
> >> >>>> implemented internally or by an external Python process, I don't mind.
> >> >>>
> >> >>> That is a harder part. (I rarely use Ctrl-Alt-2 actually; I mostly
> >> >>> use HMP on stdin).
> >> >>
> >> >> I don't think it would be that hard, actually.
> >> >>
> >> >> If you have a -qmp-shell option that takes a chardev and defaults to vc,
> >> >> you've solved the part with both stdio and Ctrl-Alt-2. Now all you need
> >> >> to do is launch the Python child process, pass it a pair of pipes for
> >> >> communication and forward everything between the pipes and the chardev.
> >> >>
> >> >> (That's the theory anyway.)
> >> > 
> >> > If someone is interested, I did a quick proof-of-concept hack:
> >> > 
> >> >     https://repo.or.cz/qemu/kevin.git/shortlog/refs/heads/qmp-shell
> >> > 
> >> > It doesn't clean up anything properly (including the qmp-shell processes
> >> > it starts), but it spawns a usable qmp-shell on a user-specified
> >> > character device. stdio seems to work, though without readline
> >> > functionality (I suppose I still have line-buffering somewhere), vc
> >> > doesn't really work at all yet.
> >> > 
> >> > Try it out like this:
> >> > 
> >> >     $ ./qemu-storage-daemon --chardev stdio,id=m --monitor 
> >> > m,mode=qmp-shell
> >> >     monitor_qmp_event: 1
> >> >     Welcome to the QMP low-level shell!
> >> >     Connected to QEMU 4.2.50
> >> > 
> >> >     (QEMU) query-version
> >> >     {"return": {"qemu": {"micro": 50, "major": 4, "minor": 2}, 
> >> > "package": "v4.2.0-1188-gd95a3885a9"}}
> >> >     (QEMU) quit
> >> > 
> >> > (Or use x86_64-softmmu/qemu-system-x86_64, but it's based on the
> >> > refactorings in the storage daemon branch, so why not try both at once?)
> >> > 
> >> > Polishing this to make it mergable would still require substantial work,
> >> > so at the moment I'm not planning to do this. But if someone wants to
> >> > pick it up, feel free (just let us know).
> >> > 
> >> > Hm, in fact... A qmp-shell GSoC project?
> >> > 
> >> 
> >> That would be great. I worry that we should have a clear vision for the
> >> syntax before we give this project to an intern, though. With a clear
> >> vision and an outline for deliverables, it's an incredibly appropriate
> >> project.
> >> 
> >> Some things I think we want to define before we start:
> >> 
> >> 1. What are we trying to achieve with a standalone shell?
> Projects without a clear goal rarely succeed.  Success within three
> months is even rarer.
> >> 2. What syntax should it use?
> Leaving that to a GSoC student amounts to setting up for failure.

I think this subthread shows that we actually have many separate
projects that people wish to have someone work on. Each of them is
probably a bit too small for a whole GSoC, but all of them together are
probably too much. So I'll guess the student would pick maybe two of
them, and if time is left at the end, more can be added as a bonus.

1. Something like --monitor mode=qmp-shell that just spawns an external
   Python script and passes it a QMP socket. This is the fundamental
   building block for having any kind of external monitor script
   actually integrated in QEMU, so I think just running the existing
   qmp-shell this way (with proper support for at least stdio and vc
   chardevs) would make sense as a first milestone.

2. Rewriting qmp-shell to use a better syntax for nested data
   structures. This would have to be defined before the project starts.

3. Improving qmp-shell UI-wise, e.g. by having better autocompletion,
   support for counting brackets, or whatever else was mentioned. We
   have a few ideas, and there's room for the student to add their own
   ideas, too.

4. Something HMP-like. This isn't QMP any more, so it could as well be a
   separate script (hmp-shell?). But it could also be integrated in
   qmp-shell in the form of additional commands that are implemented
   client-side. Or maybe have a single shell, but have a QMP mode and an
   HMP mode and the user can switch between these modes.

   The syntax for the HMP shell/mode could be the same or different from
   the QMP syntax. This would have to be defined beforehand, too.

5. Probably more that I just forgot now.

Suggesting the exact goals is part of the student application process,
but for fundamental things like the syntax we should probably already
know what we want.

> >> I think those are the hardest parts.
> >>
> >> Below, some musings:
> >> 
> >> - An integrated QMP shell would be a great usability boost to users of
> >> bare QEMU.
> >> 
> >> - It is undesirable in general to support two interfaces. Feature
> >> disparity is a problem, as is needing to document and test two separate
> >> interfaces. The quality disparity between the two is also an issue.
> >> 
> >> - Offering HMP via the GTK interface but not QMP is a discoverability
> >> problem. Unfamiliar users might assume that HMP is our flagship
> >> interface. It is not.
> >> 
> >> - We are unlikely to re-expand HMP to cover everything QMP does; writing
> >> a QMP shell that makes QMP easy to interface with is a better solution
> >> for removing redundancy and complexity.

I'm not entirely convinced of this because QMP is often too low-level to
actually address the practical high-level needs of users.

But these HMP-ish things are probably easier to maintain as scripts
outside of the QEMU binary, so I think some kind of "QMP with
extensions" for human could be the solution.

Once it's an external script, it will also be easy to exchange the shell
for another one depending on user preference, or to hack in whatever
functionality they are missing.

> >> - I suspect that the target audience for users of naked QEMU are:
> >>   - QEMU developers
> >>   - Upper-layer developers (RHV, oVirt, KubeVirt, libvirt, kata, et al)
> >> researching, testing, and debugging integration.
> >>   - Devops professionals testing, implementing and debugging
> >>     configuration & infrastructure
> >>   - Security/infosec researchers
> >>   - Embedded platform developers
> >>   - Academic researchers

Maybe kernel developers should be mentioned separately, but yes, this
list looks plausible to me.

> >> So please correct me if I am off the mark;
> >> 
> >> Design Goals:
> >>   - The removal of HMP
> >>   - An easy-to-use interface that remains reasonably "close" to the
> >> machine API such that it provides a smooth transition to scripting QEMU.
> >>   - Integration with our GTK interface for discoverability and convenience

As I listed above, I think these are actually three separate projects,
rather than goals for a single big projects.

> >> Syntax:
> >>   - TBD? Do we agree that the current syntax in qmp-shell is "bad" and
> >> should be replaced? If yes, what should it look like?
> >
> > I believe it should be a python shell with added commands.
> >
> > Simple things should be simple.
> >   e.g. adding a disk from a local file should be trivial.
> >
> > Complex things can be complex - but it would be better if they were
> > simple.
> >
> >   It's OK if the worst case of a blockdev is a bit hairy, but
> >   watch out for cases where the hairyness creeps in unnecessarily.
> Designing interfaces to complex machinery is hard.  Experience tells
> that we do okay when we focus on the building blocks first.  That's
> -blockdev.  When we start with trying to make simple things simple, we
> end in swamps.  That's -drive.
> Focus on building blocks is of course no excuse for unnecessary
> hairiness.
> It's also no reason not to build more convenient things on top of the
> building blocks.  I doubt they should go into QMP, though.

Right, they should be implemented in that external script, which would
use the lower-level QMP building blocks to provide the functionality. I
also think it's a good idea to keep QMP accessible for more exotic use
cases when the simple thing just doesn't cut it any more.

> > If the user screwsup, it should give an error that prompts the user
> > to the parameter they got wrong.
> >
> > Output from commands should normally be pretty formatted (with an option
> > to display raw json for those needing it).
> >   e.g. that 'query-version' should give either just the package
> >   version (as info version currently does) or:
> >       4.2.50  Package: v4.2.0-1188-gd95a3885a9
> >
> > We shouldn't lose any HMP commands that some people find useful
> >   Ditching HMP isn't an option until we've got almost all of it
> >   covered.
> In particular, we currently use HMP for debugging and monitoring
> purposes, where we don't need or want QMP's rigor, neither its rigorous
> interface stability, nor its structured I/O.  We want the "whipuptitude"
> we get from monitor_printf().  This is actually a point David has made
> several times.
> To have a qmp-shell replace HMP, I think it needs to be able to
> * Go beyond 1:1
>   We tried a 1:1 mapping between HMP and QMP commands, and it didn't
>   work out.  HMP's replacement should let us build convenient commands
>   from QMP building blocks.
>   We tried a 1:1 mapping between HMP and QMP command arguments, guided
>   by @args_type.  Worked out for simple cases, but was too constricting.

We need to go beyond 1:1, but we probably want to be able to offer 1:1
as a subset of commands accepted in that shell.

Offering only 1:1 in a good way might already be a step forward.

> * Preserve "whipuptitude" [David]
>   I figure that means allowing some in QMP.  Without compromising its
>   core mission, of course.

As long as we confine it to x- commands, I think this is okay.

> * As discoverable as HMP is now [Kevin]
> * Help, completion and such at least on par with what HMP provides now

Will we want to add new annotations in the schema for this?

For example, HMP has completion support for block device names. In the
QAPI schema, these are simply 'str'. We could bake the knowledge that
in command 'foo' the parameter 'bar' is a block device name, but that
would be a hack and would probably rarely be consistent with what QEMU
actually does. It's really something that schema introspection should be
able to tell us.


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