[Qemu-devel] Re: [libvirt] Libvirt debug API

["Daniel P. Berrange" <berrange@redhat.com>]

On Fri, Apr 09, 2010 at 02:16:06PM -0400, Chris Lalancette wrote:
> On 04/09/2010 10:27 AM, Daniel P. Berrange wrote:
> >> Raw access to the qemu monitor will be disabled by default; the
> >> <monitorpassthrough/> tag enables the ability to send QMP (or
> >> text, if you are using older qemu) messages straight through to the
> >> monitor.  To do this there will be an additional API entry point
> >> named virDomainDebugCommand() which takes an arbitrary string
> >> and passes it to the monitor, and returns an arbitrary string as
> >> a result.  Thus you could pass in either "info cpus" if using the
> >> text monitor or '{ "execute": "query-cpus" }' if using QMP.
> > 
> > Again the idea of a 'virDomainDebugCommand' API is QEMU specific, with
> > other hypervisors have different approaches for low level extension/
> > debug. For example, Xen would involve XenStore access, or XenD XMLRPC,
> > etc. So this should really live in a separate API namespace which is
> > specific to a hypervisor. For example, as a header file
> > 
> >   #include <libvirt/libvirt-qemu.h>
> > 
> > Containing APIs like
> > 
> >   int virDomainQEMUInvokeMonitor(virDomainPtr dom,
> >                                  const char *command,
> >                                  char **reply);
> > 
> >   typedef virConnectQEMUDomainEventCallback(virConnectPtr conn,
> >                                             virDomainPtr dom, 
> >                                             const char *eventname,
> >                                             const char *data,
> >                                             void *opaque)
> >   int virConnectQEMUDomainEventRegister(virConnectPtr conn,
> >                                         virDomainPtr dom,
> >                                         const char *eventname,
> >                                         virDomainQEMUMonitorCallback cb,
> >                                         void *opaque);
> > 
> > 
> > For an add-on library
> > 
> >   libvirt-qemu.so
> > 
> > I don't think there's much to be gained from having an XML element to
> > turn on/off use of these APIs. If an app doesn't want to use them, it
> > can simply not link to libvirt-qemu.so
> 
> The reason I wanted to do this was mostly for debug/support reasons.
> That is, with this element in place we can easily tell from the dumpxml
> output whether a person was using the "unreliable" API's, and thus we can
> tell them to try and reproduce without that in place.

That doesn't tell you whether they have actually used any API or not.
It is also inconvenient if you start a guest without it, and only later
realize you want to use the extra APIs. If we want to track the actual
usage, then the first time a direct monitor command is issued, we should
simply log a warning message. 

Daniel
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