Re: [Qemu-devel] KVM call minutes for 2013-04-23

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Apr 23, 2013 at 10:06:41AM -0600, Eric Blake wrote:
> On 04/23/2013 08:45 AM, Juan Quintela wrote:
> >   we can change "drive_mirror" to use a new command to see if there
> >   are the new features.
> 
> drive-mirror changed in 1.4 to add optional buf-size parameter; right
> now, libvirt is forced to limit itself to 1.3 interface (no buf-size or
> granularity) because there is no introspection and no query-* command
> that witnesses that the feature is present.  Idea was that we need to
> add a new query-drive-mirror-capabilities (name subject to bikeshedding)
> command into 1.5 that would let libvirt know that buf-size/granularity
> is usable (done right, it would also prevent the situation of buf-size
> being a write-only interface where it is set when starting the mirror
> but can not be queried later to see what size is in use).
> 
> Unclear whether anyone was signing up to tackle the addition of a query
> command counterpart for drive-mirror in time for 1.5.

Seems like the trivial solution is a query-command-capabilities QMP
command.

  query-command-capabilities drive-mirror
  => ['buf-size']

It should only be a few lines of code and can be used for other commands
that add optional parameters in the future.  In other words:

typedef struct mon_cmd_t {
    ...
    const char **capabilities; /* drive-mirror uses ["buf-size", NULL] */
};

> > 
> >   if we have a stable c-api we can do test cases that work. 
> 
> Having such a testsuite would make a stable C API more important.

Writing tests in Python has been productive, see qemu-iotests 041 and
friends.  The tests spawn QEMU guests and use QMP to interact:

  result = self.vm.qmp('query-block')
  self.assert_qmp(result, 'return[0]/inserted/file', target_img)

Using this XPath-style syntax it's very easy to access the JSON.

QEMU users tend not to use C, except libvirt.  Even libvirt implements
the QMP protocol dynamically and can handle optional arguments well.

I don't think a static C API makes sense when we have an extensible JSON
protocol.  Let's use the extensibility to our advantage.

Stefan