Re: Making QEMU easier for management tools and applications

[Michal Prívozník]

On 12/21/19 10:02 AM, Markus Armbruster wrote:
> Stefan Hajnoczi <address@hidden> writes:
> 


>> 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.

This is something that bothered me for a while now. Even though it's not
as bad as it used to be because we are not adding so much wrappers for
monitor commands as we used to. I mean, in libvirt the wrapper for a
monitor command has to be written by hand. Worse, whenever I'm adding a
wrapper I look at the QMP schema of it and let my muscle memory write
the wrapper.

However, it's not only what Markus already mentioned. Even if we
generated wrappers by a script, we need to be able to generate wrappers
for every single supported version of qemu.

For instance, if qemu version X has a command that accepts some set of
arguments and this set changes in version X+1 then libvirt needs both
wrappers and decides at runtime (depending on what version it is talking
to) what wrapper to use.

Unfortunately, I don't see any easy way out.

Michal