Re: Making QEMU easier for management tools and applications

[Stefan Hajnoczi]

On Sat, Dec 21, 2019 at 10:02:23AM +0100, Markus Armbruster wrote:
> Stefan Hajnoczi <address@hidden> writes:
> 
> > Hi,
> > QEMU presents a command-line interface and QMP monitor for
> > applications to interact with.  Applications actually need API
> > bindings in their programming language.  Bindings avoid reimplementing
> > code to spawn a QEMU process and interact with QMP.  QEMU is kind of
> > lazy and de facto relies on libvirt for API bindings.
> >
> > Is it time for better QEMU APIs?
> >
> > 1. We have qapi-schema.json.  Let's render to HTML and publish
> > versioned documentation online.
> 
> Make can build docs/interop/qemu-qmp-ref.{7,html,info,pdf,txt}.  Grab
> the .html and go for it.  There's also qmp-ga-ref.

The missing step here is to integrate doc generation and upload into the
release process so the documentation is published.

Once my recent documentation publishing patches have been merged I'll
see if Mike Roth wants to extend the release scripts.

> > 2. scripts/qmp/ contains command-line tools for QMP communication.
> > They could use some polish and then be shipped.
> 
> MAINTAINERS blames them on me, but they're effectively unmaintained.
> Prerequisite for shipping: having a maintainer who actually gives a
> damn.
...
> * scripts/qmp/qmp-shell
> 
>   Half-hearted attempt at a human-friendly wrapper around the JSON
>   syntax.  I have no use for this myself.

I think this one is used by people.  John Snow comes to mind.

> > 3. python/qemu/ contains Python modules for managing a QEMU process
> > and QMP communication.  This should be packaged in distros and
> > available on PyPI.
> 
> Currently maintained by Eduardo and Cleber (cc'ed) under "Python
> scripts".
> 
> > 4. Go and Rust bindings would also be useful.  There is
> > https://github.com/intel/govmm but I think it makes sense to keep it
> > in qemu.git and provide an interface similar to our Python modules.
> 
> Mapping QAPI/QMP commands and events to function signatures isn't hard
> (the QAPI code generator does).  Two problems (at least):
> 
> 1. Leads to some pretty ridiculous functions.  Here's one:
> 
>     void qmp_blockdev_mirror(bool has_job_id, const char *job_id,
>                              const char *device,
>                              const char *target,
>                              bool has_replaces, const char *replaces,
>                              MirrorSyncMode sync,
>                              bool has_speed, int64_t speed,
>                              bool has_granularity, uint32_t granularity,
>                              bool has_buf_size, int64_t buf_size,
>                              bool has_on_source_error,
>                              BlockdevOnError on_source_error,
>                              bool has_on_target_error, BlockdevOnError 
> on_target_error,
>                              bool has_filter_node_name, const char 
> *filter_node_name,
>                              bool has_copy_mode, MirrorCopyMode copy_mode, 
>                              bool has_auto_finalize, bool auto_finalize,
>                              bool has_auto_dismiss, bool auto_dismiss,
>                              Error **errp);
> 
>   We commonly use 'boxed': true for such beasts, which results in
>   functions like this one:
> 
>     void qmp_blockdev_add(BlockdevOptions *arg, Error **errp);
> 
> 2. Many schema changes that are nicely backward compatible in QMP are
>    anything but in such an "obvious" C API.  Adding optional arguments,
>    for instance, or changing integer type width.  The former is less of
>    an issue with 'boxed': true.
> 
> Perhaps less of an issue with dynamic languages.
> 
> I figure a static language would need much more expressive oomph than C
> to be a good target.  No idea how well Go or Rust bindings can work.

Most of what govmm does is build QEMU command-lines, not wrap QMP APIs.

Native QMP API bindings are harder and we'd need to see who really needs
this.  Languages with a built-in dict type like Python make working with
JSON-level QMP feasible.  In C it's painful though and native API
bindings are desirable.

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