Re: Modern HMP

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

On Wed, Feb 04, 2026 at 09:08:49AM +0100, Markus Armbruster 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.

Surely the isolated HMP monitor code can just keep track of its view
of a "default" CPU, and then pass an explicit CPU to the QMP commands
it runs ?


With regards,
Daniel
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