Re: Modern HMP

["Dr. David Alan Gilbert" <dave@treblig.org>]

* Markus Armbruster (armbru@redhat.com) wrote:
> "Dr. David Alan Gilbert" <dave@treblig.org> writes:
> 
> > * Daniel P. Berrangé (berrange@redhat.com) wrote:
> >> On Thu, Jan 22, 2026 at 12:47:47PM -0300, Fabiano Rosas wrote:
> >> > One question I have is what exactly gets (eventually) removed from QEMU
> >> > and what benefits we expect from it. Is it the entire "manual"
> >> > interaction that's undesirable? Or just that to maintain HMP there is a
> >> > certain amount of duplication? Or even the less-than-perfect
> >> > readline/completion aspects?
> >> 
> >> Over time we've been gradually separating our human targetted code from
> >> our machine targetted code, whether that's command line argument parsing,
> >> or monitor parsing. Maintaining two ways todo the same thing is always
> >> going to be a maint burden, and in QEMU it has been especially burdensome
> >> as they were parallel impls in many cases, rather than one being exclusively
> >> built on top of the other.
> >> 
> >> Even today we still get contributors sending patches which only impl
> >> HMP code and not QMP code. Separating HMP fully from QMP so that it
> >> was mandatory to create QMP first gets contributors going down the
> >> right path, and should reduce the burden on maint.
> >
> > Having a separate HMP isn't a bad idea - but it does need some idea of
> > how to make it easy for contributors to add stuff that's just for debug
> > or for the dev.   For HMP the bar is very low; if it's useful to the
> > dev, why not (unless it's copying something that's already in the QMP interface
> > in a different way);  but although the x- stuff in theory lets
> > you add something via QMP, in practice it's quite hard to get it through
> > review without a lot of QMP design bikeshedding.
> 
> I think this description has become less accurate than it used to be :)
> 
> A long time ago, we started with "QMP is a stable, structured interface
> for machines, HMP is a plain text interface for humans, and layered on
> top of QMP."  Layered on top means HMP commands wrap around QMP
> commands.  Ensures that QMP is obviously complete.  Without such a
> layering, we'd have to verify completeness by inspection.  Impractical
> given the size and complexity of the interfaces involved.
> 
> Trouble is there are things in HMP that make no sense in QMP.  For
> instance, HMP command 'cpu' sets the monitor's default CPU, which
> certain HMP commands use.  To wrap 'cpu' around a QMP command, we'd have
> to drag the concept "default CPU" into QMP where it's not wanted.

That's just state; that's easy!

> So we retreated from "all", and permitted exceptions for commands that
> make no sense in QMP.
> 
> We then found out the hard & expensive way that designing a QMP command
> with its stable, structured interface is often a lot harder than
> cobbling together an HMP command.  It's not just avoidable social
> problems ("bikeshedding"); designing stable interfaces is just hard.
> Sometimes the extra effort is worthwhile.  Sometimes it's not, e.g. when
> all we really want is print something to aid a human with debugging.

Right.

> So we retreated from "all" some more, and permitted exceptions for
> commands meant exclusively for human use, typically debugging and
> development aids.
> 
> This effectifely redefined the meaning of "complete": instead of "QMP
> can do everything HMP can do and more", it's now "... except for certain
> development and debugging aids and maybe other stuff".
> 
> To keep "maybe other stuff" under control, we required (and still
> require) an *argument* for adding functionality just to HMP.
> 
> This turned out to be differently bothersome.  Having to review HMP
> changes for QMP bypasses is bothersome, and bound to miss things at
> least occasionally.  Having to ask for an argument is bothersome.
> Constructing one is bothersome.

Well, it's not too bad as long as the arguments are only asked to be
reasonable rather than bulletproof.

> To reduce the bother, we retreated from another QMP ideal: the
> structured interface.  Permit QMP commands to return just text when the
> command is meant just for humans.  Such commands must be unstable.
> Possible because we had retreated from "all of QMP is stable" meanwhile.
> 
> How does this work?  Instead of adding an HMP-only command, add a QMP
> command that returns QAPI type HumanReadableText, and a trivial HMP
> command that wraps around it.  Slightly more work, but no interface
> design.

Yes, I've seen that - and as long as that continues it's OK;
although it does feel weird in a few ways; it seems more work
to implement than just being able to print, but I do worry
what happens with commands with a lot of output.   It also gets
weirder when you have to parameterise it, because you get the parameter
parsing in one place influencing the separate formatting.

> The QMP command addition is much more visible to the QAPI/QMP
> maintainers than an HMP-only command would be.  This helps avoid missing
> things.
> 
> We still want an argument why a structured interface isn't needed.  But
> we can be much more lenient there: if it turns out to be needed, we can
> just add it, and drop the unstructured interface.  Remember, it's
> unstable.
> 
> Hope this helps.

Yeh, it's just something to be watchful of.

I did see you suggesting for Rust for it; which would work - although
given it wouldn't be performance sensitive, Python would seem reasonable.

Dave

Dave

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