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

From: Anthony Liguori
Subject: Re: [Qemu-devel] KVM call minutes for Mar 15
Date: Tue, 15 Mar 2011 16:05:32 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110223 Lightning/1.0b2 Thunderbird/3.1.8

On 03/15/2011 02:06 PM, Chris Wright wrote:
* Anthony Liguori (address@hidden) wrote:
On 03/15/2011 09:53 AM, Chris Wright wrote:
- 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:

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
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.


Anthony Liguori


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