On Tue, Jan 11, 2011 at 11:10:04AM +0000, Daniel P. Berrange wrote:
> On Tue, Jan 11, 2011 at 11:46:14AM +0100, Gerd Hoffmann wrote:
> > On 01/10/11 18:19, Cole Robinson wrote:
> > >In QEMU, the card itself is a PCI device, but it requires
> > >-device hda-output in order to actually get sound to the host. AIUI this
> > >is just HDA configuration and does not require stable addressing, so
> > >is not presently represented in the XML.
> >
> > It isn't that simple. There are actually multiple devices involved.
> > Each audio codec (yes, there can be multiple of these, up to 15) is
> > connected via HDA Link to the pci controller. Each audio codec has
> > a codec address (HDA bus property: cad=[0..14]).
> >
> > So you can specify "-device intel-hda -device hda-duplex -device
> > hda-output" and the guest has multiple audio devices. Win7 even
> > handles this correctly, whereas alsa uses only the first codec it
> > finds (lowest codec address). Not that this buys you much today,
> > qemu mixes all channels together before sending them off to the
> > hosts sound system, i.e. you don't see a stream per sound card in
> > pulseaudio. That might change in the future though.
> >
>
> So 'intel-hda' should really be considered as a controller,
> and hda-output' & 'hda-duplex' are the things to be treated
> as devices in the guest config.
>
> This suggests a setup more like the one we did for virtio-serial
> where we'd invent a new address type for codecs, and have XML
> looking something like
>
> <controller type='hda' model='ich6'>
> <address type='pci' domain='0x0000' bus='0x00' slot='0x0a'
> function='0x0'/>
> </controller>
> <sound model='hda-output'>
> <address type='hda' codec='0'/>
> </sound>
> <sound model='hda-duplex'>
> <address type='hda' codec='1'/>
> </sound>
>
> Are there any technical model names for 'hda-output' 'hda-duplex'
> we need to be aware of (as we potentially have ich6, ich7, 8, 9,
> etc for intel-hda in the future)
>
> > We also might see more hda codecs in the future. Maybe to pass
> > through digital audio interfaces from the host? Not investigated
> > yet in detail though.
> >
> > The HDA Links are even hot-pluggable (new audio codec can be linked
> > up when putting the laptop into the docking station for example),
> > although the qemu emulation doesn't support that today.
>
> Hotplug is another good reason for us to treat the 'hda-output'
> and 'hda-duplex' objects as the <sound> device in libvirt XML,
> because then we can add/remove them via virDomainAttachDevice
> which we couldn't otherwise do
One further alternative is to explicitly list codecs under the
<sound> element eg
<sound model='ich6'>
<codec name='output' slot=0/>
<codec name='duplex' slot=3/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x0a' function='0x0'/>
</sound>
We could use virDomainAttachDevice/DetachDevice to hotplug an
entire ich6 device, and virDomainUpdateDevice to hotplug
individual codecs within the device. This might be a slightly
nicer option for applications, so it would more closely mesh
with what the guest OS actually sees as its device(s).
Daniel
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