Re: [Qemu-devel] KVM call minutes for Mar 15

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/15/2011 02:06 PM, Chris Wright wrote:
> * Anthony Liguori (anthony@codemonkey.ws) wrote:
>> On 03/15/2011 09:53 AM, Chris Wright wrote:
>>> QAPI
> <snip>
>>> - c library implementation is critical to have unit tests and test
>>>    driven development
>>>    - thread safe?
>>>      - no shared state, no statics.
>>>      - threading model requires lock for the qmp session
>>>    - licensiing?
>>>      - LGPL
>>>    - forwards/backwards compat?
>>>      - designed with that in mind see wiki:
>>>
>>>        http://wiki.qemu.org/Features/QAPI
>> One neat feature of libqmp is that once libvirt has a better QMP
>> passthrough interface, we can create a QmpSession that uses libvirt.
>>
>> It would look something like:
>>
>> QmpSession *libqmp_session_new_libvirt(virDomainPtr dom);
> Looks like you mean this?
>
>         ->  request QmpSession ->
> client                          libvirt
>         <- return QmpSession<-
>
> client ->  QmpSession ->  QMP ->  QEMU

Maybe, your ASCII art confuses me :-)

QmpSession is just a wrapper around a transport.  It can be an fd that 
you read() and write() JSON strings to, but it's just as easy to read 
and write through JSON strings via virQemuMonitorCommand() or whatever 
the interface currently is.

> So bypassing libvirt completely to actually use the session?
>
> Currently, it's more like:
>
> client ->  QemuMonitorCommand ->  libvirt ->  QMP ->  QEMU

It's not bypassing.  It's an API on top of a libvirt command.

FWIW, the code generator could be trivially modified to generate a 
libvirt style API if there's any interest.  So instead of:

void qmp_block_passwd(QmpSession *sess, const char *device, const char 
*password, Error **errp);

It would be:

int virQemuBlockPasswd(virDomainPtr dom, const char *device, const char 
*password);

But I'm not sure that's really that useful.

>> The QmpSession returned by this call can then be used with all of
>> the libqmp interfaces.  This means we can still exercise our test
>> suite with a guest launched through libvirt.  It also should make
>> the libvirt pass through interface a bit easier to consume by third
>> parties.
> This sounds like it's something libvirt folks should be involved with.
> At the very least, this mode is there now and considered basically
> unstable/experimental/developer use:
>
>   "Qemu monitor command '%s' executed; libvirt results may be unpredictable!"
>
> So likely some concern about making it easier to use, esp. assuming
> that third parties above are mgmt apps, not just developers.

To be clear, there's no real support needed from libvirt here other than 
the passthrough.

How that interface is supported in libvirt is more or less orthogonal to 
libqmp.  libqmp is a C API to QMP.  It can speak QMP over whatever 
transports speak QMP.  If you can speak QMP to libvirt, then it's only 
natural to bridge libqmp to libvirt.

Regards,

Anthony Liguori

> thanks,
> -chris
>