[pve-devel] [PATCH qemu-server] audio dev: specify qemu audio backend

[Thomas Lamprecht]

On 7/22/19 12:33 PM, Aaron Lauterer wrote:> On 7/22/19 10:00 AM, Thomas Lamprecht wrote:
>> On 7/19/19 3:15 PM, Aaron Lauterer wrote:
>>> QEMU 4 adds the `-audiodev` parameter to explicitly specify the audio
>>> backend. Setting it avoids occasional error messages when starting a
>>> virtual machine with an audio device and qemu wants to connect it to the
>>> physical audio device.
>>
>> Occasional? The error is always there if no audio card/alsa is there.
>> The real issue is that this would have allowed access to host HW with
>> only having the VM.Config.Option permission on any VM..
>> (no big issue, I just like to have the commit messages tell the real reasons)
>>
> 
> Managed to find a reproducible setup to cause the errors. Will rewrite the commit msg.
> 
>>>
>>> For now only SPICE is supported as it's also the biggest use case.
>>
>>
>> FYI, testing this in a VM I get the following errors/warnings:
>>
>>> audio: Could not init `spice' audio driver
>>> audio: warning: Using timer based audio emulation
>>
>> but host seems to work...
>>
> 
> These errors are thrown if the "-audiodev spice" parameter is set but no spice (qxl) display is selected. If no spice display is selected the "-spice" parameter is not set.
> 
> The manpage for qemu states the following regarding "-audiodev spice":
> 
>> Creates a backend that sends audio through SPICE.  This backend requires "-spice" and automatically selected in that case, so usually you can ignore this option.  This backend has no backend specific properties.
> 
> If spice(qxl) is selected as display there are no errors even if the "-audiodev spice" is not set.
Ah okay, makes even (somewhat) sense, thanks for clearing this up!

> 
> To limit access to the host hardware I would still set the "-audiodev spice" for the cases that a user defines an audio device but does not set the display to spice(qxl).
> 
> What do you think? @Thomas

Yes, sure that always should be done, else it would use the alsa host stuff.
Real questions is if we should allow setting the audio0 then when qxl is
not the display - I mean in the backend yes, for sure (maybe warn on startup
then) but in the WebUI it could really make sense to merge the audio dev
addition into the SPICE/QXL display edit?

> I could not find any doc regarding the need for a matching id or a different behavior. If someone can shed more light onto this I would appreciate it.

>From the offending commit[0] adding this pesky audiodev I still interpret
that this is really to configure specific audio drivers (the intel-hda, et
al. stuff in QEMU speak), and normally to combine those and ID is needed,
that's the case with most devices + their configuration in QEMU. So I
/guess/ that the id has it's purpose and that this is it..

I guess that it here only still works because in the with QXL as display it
defaults here to it?

Anyway, I'm not too happy with this all if we do not have an idea how it
works and what really is affected by this. Do you think you can ask around
in the IRC or user mailing list, CCing the patch author could be an option,
albeit a bit intrusive one..
I'd be much happier if we had a bit clarity here, not just guess-work,
that's no accusation, the docs really do not makes ones life easy here, and
while the code helps - I currently just do not have the time to make me
familiar with it and see what really happens and if that's OK..

[0]: https://git.qemu.org/gitweb.cgi?p=qemu.git;a=commitdiff;h=71830221fb23388b32e6516c2fb7a698453a6c5a