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Re: [libvirt] [PATCH] qemu: Allow using regular audio backends with VNC



On 05/24/2010 10:31 AM, Daniel P. Berrange wrote:
> On Mon, May 24, 2010 at 10:26:43AM -0400, Cole Robinson wrote:
>> On 05/24/2010 10:14 AM, Daniel P. Berrange wrote:
>>> On Thu, May 20, 2010 at 12:04:04PM -0400, Cole Robinson wrote:
>>>> Currently all host audio backends are disabled if a VM is using VNC, in
>>>> favor of the QEMU VNC audio extension. Unfortunately no released VNC
>>>> client supports this extension, so users have no way of getting audio
>>>> to work if using VNC.
>>>>
>>>> Add a new config option in qemu.conf which allows changing libvirt's
>>>> behavior, but keep the default intact.
>>>>
>>>> Signed-off-by: Cole Robinson <crobinso redhat com>
>>>> ---
>>>>  src/qemu/qemu.conf   |   10 ++++++++++
>>>>  src/qemu/qemu_conf.c |   17 ++++++++++++-----
>>>>  src/qemu/qemu_conf.h |    2 ++
>>>>  3 files changed, 24 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/src/qemu/qemu.conf b/src/qemu/qemu.conf
>>>> index 3da332f..fec946d 100644
>>>> --- a/src/qemu/qemu.conf
>>>> +++ b/src/qemu/qemu.conf
>>>> @@ -168,3 +168,13 @@
>>>>  # be assigned to guests.
>>>>  #
>>>>  # relaxed_acs_check = 1
>>>> +
>>>> +
>>>> +# QEMU implements anextension for providing audio over a VNC connection,
>>>> +# though if your VNC client does not support it, your only chance for getting
>>>> +# sound output is through regular audio backends. By default, libvirt will
>>>> +# disable all QEMU sound backends if using VNC, since they can cause
>>>> +# permissions issues. Enabling this option will make libvirtd honor the
>>>> +# QEMU_AUDIO_DRV environment variable when using VNC.
>>>> +#
>>>> +# vnc_enable_audio_backend = 0
>>>
>>> I think this would be better named as 'vnc_allow_host_audio'
>>> since its not toggling VNC audio, just whether it is allowed
>>> to use the host audio drivers. 
>>>
>>
>> Okay, I'll change and resend.
>>
>>> NB, even with this toggled things are unlikely to magically
>>> 'just work'. If QEMU is running as 'qemu' user ID, that user
>>> won't have any permissions to use /dev/snd/* devices. And
>>> if running as 'root' then pulseaudio won't autospawn so will
>>> need to be run manually.
>>>
>>
>> Right. Currently getting sound to work in Fedora is a big pain, and
>> there have been quite a few user complaints. I'm going to write up a
>> 'known bug' page, documenting a workaround of qemu.conf user/group =
>> $USER to fix the issue, but this patch will still need to be backported.
> 
> I'd really recommend against telling people to configure libvirt to run
> the guests as their own $USER because that'll open a huge can of worms.
> 

What exactly are the downsides, besides the security implications? It
should exercise all the same code as user/group = 'qemu', but solves
issues like:

- Making sure emulator user can access $HOME for install media
- Audio
- PTY access
- SDL (if someone insists on it)

> Better to tell them to add an ACL to /dev/snd/* using setfacl to add the
> qemu user to the ACL for the sound card, or drop in a udev rule todo the
> same.
> 

I'll try that out, but I wonder will my local user actually hear that
sound? Will the audio go to my pulseaudio session?

- Cole


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