Re: Making QEMU easier for management tools and applications

[Daniel P . Berrangé]

On Wed, Jan 15, 2020 at 01:15:17PM +0100, Markus Armbruster wrote:
> Christophe de Dinechin <address@hidden> writes:
> 
> > Thanks a bunch. This clarifies a number of my misconceptions about
> > how this is currently used. Most notably this one:
> >
> >> On 15 Jan 2020, at 10:20, Markus Armbruster <address@hidden> wrote:
> >> 
> >>> We don’t want the QAPI to let arbitrary fields of a QOM object
> >>> be modified, do we?
> >> 
> >> We already do: QMP command qom-set.  If it breaks your guest, you get to
> >> keep the pieces.
> >
> > Ouch. I certainly did not expect that.
> >
> > "It is not what you don’t know that kills you, it is what you know that 
> > ain’t so".
> 
> Do we have a legitimate use for qom-set right now?  Hmm, let's check
> libvirt... aha:
> 
> * qemuMonitorJSONSetMemoryStatsPeriod() uses it to control
>   virtio-balloon's guest-stats-polling-interval property, in accordance
>   with docs/virtio-balloon-stats.txt.
> 
> * qemuMonitorJSONSetIOThread() uses it to control iothread's properties
>   poll-max-ns, poll-grow, poll-shrink.  Their use with -object is
>   documented (in qemu-options.hx), their use with qom-set is not.
> 
> Oh well.

Libvirt is of course happy to switch to something else instead of
qom-set for these features if QEMU wants to provide a safer
alternative.


Regards,
Daniel
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