Re: qmp-shell for GSoC/Outreachy?

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Feb 06, 2020 at 10:40:37AM +0100, Markus Armbruster wrote:
> > 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.

I'd like to argue that this decision to keep these debugging/monitoring
things in HMP only was a mistake, because it ensures that QEMU internals
need to keep HMP related code forever.

What we actually need is a part of QMP that does not have the long term
stability requirement, nor need for fully structured data. In fact this
pretty much already exists - we have declared the 'x-' prefix as a way
to model QMP commands which are experimental / suboptimal / subject
to change.

I suggest that every HMP command which does not have a QMP equivalent
should be turned into a QMP command with an "x-" prefix, with no
extra modelling applied

Take "info block"

(hmp) info block
ide1-cd0: [not inserted]
    Attached to:      /machine/unattached/device[23]
    Removable device: not locked, tray closed

floppy0: [not inserted]
    Attached to:      /machine/unattached/device[16]
    Removable device: not locked, tray closed

sd0: [not inserted]
    Removable device: not locked, tray closed


I suggest we support it as "x-query-block"

(qmp) x-query-block
{
    "return": {
        "info": "ide1-cd0: [not inserted]
    Attached to:      /machine/unattached/device[23]
    Removable device: not locked, tray closed

floppy0: [not inserted]
    Attached to:      /machine/unattached/device[16]
    Removable device: not locked, tray closed

sd0: [not inserted]
    Removable device: not locked, tray closed"
    }
}


Functionally we in fact already support pretty much exactly
that via the "human-monitor-command"  QMP command.

The difference is that with the latter, we will still have to
keep around the internal dispatching machinery for HMP inside
QEMU forever. If we transplant all remaining HMP commands with
an "x-" prefix, we open up the possibility of completely
separating HMP out and having QEMU work exclusively with QMP
internally.

This is complementary to an improved qmp-shell client.

Regards,
Daniel
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