On 12/21/19 10:02 AM, Markus Armbruster wrote: > Stefan Hajnoczi <stefa...@gmail.com> 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
