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Re: [Qemu-devel] [HACK] hda: expose microphone instead of line-in

From: Alexander Graf
Subject: Re: [Qemu-devel] [HACK] hda: expose microphone instead of line-in
Date: Thu, 3 May 2012 00:42:58 +0200

On 02.05.2012, at 20:17, Jan Kiszka wrote:

> On 2012-04-25 09:34, Gerd Hoffmann wrote:
>> On 04/25/12 13:03, Jan Kiszka wrote:
>>> Hi Gerd,
>>> I had problems with Windows LiveMeeting expecting a microphone as
>>> input. But the HDA model only exposes a line-in port. The following hack
>>> works for me, but I bet there is a cleaner solution. Any suggestions?
>> Good to know this works.  /me has patches ready to go, was just waiting
>> for testing feedback ...
>> Pushed to git://git.kraxel.org/qemu audio.1
>> They do essentially the same, except that they leave the existing
>> hda-duplex code as-is and add a new hda-micro codec instead which
>> advertises the input as micro to the guest.
> Yep, would work fine - if the issue below allowed me to use it.
>>> BTW, sound output quality of a Win7 guest on my Linux hosts sucks while
>>> it's fine for a Linux guest. I vaguely recall that Windows requests a
>>> too small DAC buffer, is that true? Is there anything one can do about
>>> this?
>> Yes.  The buffer is ~ one page and can hold 20 ms of sound data, so
>> considering buffer flipping intel-hda has to shuffle data every 10ms,
>> and the windows guest needs to be scheduled too so it can re-fill the
>> other half of the buffer.  Which obviously makes sound playback *very*
>> sensitive to latencies anywhere in the qemu.
>> What you can do about it?  Dunno whenever windows allows to tweak the
>> buffer size somehow.  When I looked deeper at that a while back the
>> biggest latency issues in qemu used to be qxl, ide/qcow2 and vnc.  qcow2
>> should be fixed now with the switch to coroutines and full async i/o.
>> Likewise qxl, although this depends on recent guest drivers.  For vnc
>> enabling the threaded vnc server helps alot (without it moving around
>> windows leads to sound dropouts).
> I found another workaround: audio hw passthrough. Works nicely. And this
> indicates that there should be still some room for improvement in the
> device model so that Windows chooses a proper ring buffer size, no?

Why? For hw passthrough, mmio doesn't go through qemu anymore, right? :)


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