Re: [Qemu-devel] [PATCH v2 4/4] audio: paaudio: ability to specify stream name

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

On Wed, Aug 28, 2019 at 01:14:03PM +0300, Maxim Levitsky wrote:
> On Wed, 2019-08-28 at 10:53 +0100, Daniel P. Berrangé wrote:
> > On Wed, Aug 28, 2019 at 12:43:49AM +0200, Zoltán Kővágó wrote:
> > > On 2019-08-27 07:42, Gerd Hoffmann wrote:
> > > > On Mon, Aug 26, 2019 at 09:59:04PM +0200, Kővágó, Zoltán wrote:
> > > > > This can be used to identify stream in tools like pavucontrol when one
> > > > > creates multiple -audiodevs or runs multiple qemu instances.
> > > > 
> > > > Hmm, can we create an useful name automatically, without yet another
> > > > config option?
> > > > 
> > > > Useful choices could be the device name (usb-audio, ...) or the device
> > > > id (whatever -device id=xxx was specified on the command line).
> > > 
> > > I'm afraid this is not going to work with the current architecture: due
> > > to mixeng even if you have multiple devices, they'll be mixed to a
> > > single stream and the audio backend will only see this one mixed stream.
> > >  As a workaround we could do something like concat all device names or
> > > ids, but I don't like that idea.
> > > 
> > > Alternatively we could use the id of the audiodev instead, and no more
> > > problems with mixeng.  However, with mixeng off (implemented in my next
> > > patch series) suddenly soundcards will have suddenly end up as different
> > > streams.  (This can be worked around by creating multiple audiodevs,
> > > like what you have to use now to get multiple streams from pa, so this
> > > is probably a smaller problem.)
> > > 
> > > Currently I'm leaning for the audiodev's id option, unless someone
> > > proposes something better.
> > 
> > Using the audiodev id is not a good idea. If you have multiple QEMU's
> > on your host, it is highly likely that libvirt will have assigned
> > the same audiodev id to all of them.  Using the vm name would be ok,
> > but only if you assume that each gust only has a single audio device.
> > 
> > Using a combination of vm name + audidev id is going to be unique
> > per host, but not especially friendly as a user visible name. It
> > would be ok as a default, but I'd think we should let the mgmt app
> > specify stream name explicitly, so that something user friendly
> > can be set.
> No, no!
> It seems that pulseaudio has a name for each connection, and a name for each
> steam within that connection.
> 
> The suggestion is that we use the VM name for the connection,
> (which will be unique per VM usually, at least the user can make it be so)
> and then use the audiodev id for each stream. Of course for multiple VMs,
> the audiodev ids will be the same, but this is all right since you can
> always distinguish them that the streams come from different VMs.

Ok, if I'm reading the code correctly, it seems we do take care to
re-use a single connection to PA for all audiodevs we create. So a
VMname is fine for the connection.

> Also note that this thing is cosmetic from the correctness point of view,
> that is pulse-audio internally has no problem with duplicate IDs.
> 
> The thing is useful mostly for tweaking the output streams in the pavucontrol,
> where the names will allow you to easily know which steam is which.

Yep, I wasn't really concerned about internals - from the user POV being
able to accurately distinguish streams in pavucontrol is very important
though, so we should ensure that's possible. If we use 'id'  for the
stream as a default though, we should still allow an override, as 'id'
values are not really intended as end user visible data. If a guest
has multiple devices I'd expect to be able to give them names that are
meaningful to me as a user, not something libvirt auto-generates for
its own machine oriented use.

Regards,
Daniel
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