Re: [PATCH 7/8] qmp: add filtering of statistics by name

[Paolo Bonzini]

On 4/27/22 19:16, Dr. David Alan Gilbert wrote:
> * Paolo Bonzini (pbonzini@redhat.com) wrote:
> > On 4/27/22 17:16, Dr. David Alan Gilbert wrote:
> > > * Paolo Bonzini (pbonzini@redhat.com) wrote:
> > > > On 4/27/22 14:34, Dr. David Alan Gilbert wrote:
> > > > > If I specify a 'vm' it's not obvious to me whether I'd get NICs and
> > > > > block devices in the future?
> > > >
> > > > VM would not get those (it's global statistics), but the size could balloon
> > > > if you specify no target at all.
> > > >
> > > > > Adding a syntax for 'all' into the vcpus list would fix that?
> > > >
> > > > I don't like having special syntax.  The current QAPI just doesn't filter
> > > > what is not in the arguments.
> > >
> > > Is there a object that represents the set of all vcpus?
> >
> > No.
>
> If it was easy to create one then you could remove all the special
> casing of vCPUs/VM target?
> (It feels really like you should call a 'stats' method on the target)

There are two possibilities for that:

1) add statistics to an object like /machine, that would return the 
sum/max of the statistics.  Advantage: you have an easy way to summarize 
stats without reading many KBs of data. Disadvantage: it doesn't do what 
you're asking. :)  But it may be an interesting addition.

2) make query-stats return the statistics for all objects below a given 
QOM path, and then the caller would pass / or /machine as the target.

Both make sense, but neither extends easily to the case where you don't 
have a QOM path, as is the case for block or network devices. 
Unfortunately, both of them are prime candidates for extending the 
subsystem, so they can't be dismissed easily, and that is why 
implemented neither of them.

If block or network devices were QOM, it would be possible and easy to 
have a single "qom-path" argument to replace both "target" and the 
sub-records like StatsVCPUFilter.

Paolo