[libvirt-users] Audio in Windows 10 VM is distorted. Using ALSA.

[Žilvinas Žaltiena]

Hi,

I found the main reason for sound distortions on my system is guest timer
configuration.

The working one is:

<clock offset="localtime">
    <timer name="hypervclock" present="yes"/>
    <timer name="hpet" present="no"/>
    <timer name="rtc" present="yes" track="guest" tickpolicy="delay"/>
</clock>
...
<qemu:commandline>a
    <qemu:arg value='-machine'/>
    <qemu:arg value='kernel_irqchip=on'/>
    <qemu:arg value='-cpu'/>
    <qemu:arg
value='host,migratable=no,+invtsc,hv_time,+kvm_pv_eoi,hv_relaxed,hv_spinlocks=0x1fff,hv_vendor_id=nv_fuck_you!,kvm=off'/>
</qemu:commandline>

The important part here is "host,migratable=no,+invtsc". It gives guest the
invariant TSC (CPU must support it). This is a bit of a hack. Everything,
except migratable=no, can be configured through proper XML elements,
however +invtsc requires migratable=no, and I don't know a way to pass it
other than using qemu:arg. Note it completely overrides qemus -cpu
parameter created from other tags. Don't paste my qemu:arg of it. Run your
VM without it, check libvirt log, and copy from it what is passed to qemu
as -cpu parameter, then add migratable=no,+invtsc.

2016-12-20 4:41 GMT+02:00 Manuel Ullmann <ullman.alias at posteo.de>:

> Hi,
>
> I can’t seem to find a solution for my last VM issue.
> I have distorted sound, when I play the Windows 10 device test
> sound. Only the Windows sound is affected. Mpd of the host is playing just
> fine
> meanwhile. I am using ich9 as emulated card, which is detected and
> installed correctly. The VM is using vfio-igd passthrough, so  vnc or
> spice are not used.
> I tried to match the Windows driver configuration, which only supports
> 16 bit output. I have a 24 bit USB headphone amplifier, which does not
> support it (S32_LE only). Trying direct playback on the PCH card jack
> did not help though.
>
> The headphone amplifier is connected via optical Toslink, so there is
> standard snd_hda_intel output used (snd_usb_audio is not involved).
> Passing the USB part through I get much clearer output, but still have
> popping sounds. Googling around, they seem to be quite common on Windows
> 10, although I had it working fine on two bare-metal machines.
>
> I have tried fixed settings matching the ones of the dmixed device
> too. Don’t know, what is polled with enabled poll option. I also tried
> sdl and pulseaudio, which did not help. For the latter not so surprising
> since it is an Alsa pipe. The former was worse.
>
> The goal of this setup is having the same playlist state on both
> systems and using the better audio framework for music playback (Windows
> uses
> too much software layers). Of course the popping sounds on audio device
> passthrough also would be annoying, but that might be a different issue.
>
> Best regards,
> Manuel
>
> used alsa device:
> pcm.dmixed {
>         type asym
>         playback.pcm {
>                 # See plugin:dmix at http://www.alsa-project.org/
> alsa-doc/alsa-lib/pcm_plugins.html
>                 type dmix
>
>                 # Don't block other users, e.g. the Timidity midi-player
> daemon
>                 # http://www.alsa-project.org/
> alsa-doc/alsa-lib/pcm_plugins.html
>                 ipc_key_add_uid false
>
>                 ipc_key 1025
>                 ipc_perm 0660
>                 ipc_gid audio
>
>                 # Don't put the rate here! Otherwise it resets the rate &
> channels set below, as shown by:  cat /proc/asound/card0/pcm0p/sub0/
> hw_params
>                 slave {
>                         # 2 for stereo, 6 for surround51, 8 for surround71
>                         channels 2
>                         pcm {
>                                 format S16_LE
>                                 #format S32_LE
>
>                                 #rate 44100
>                                 rate 48000
>
>                                 # http://www.alsa-project.org/
> alsa-doc/alsa-lib/pcm_plugins.html
>                                 # Maybe helps
>                                 nonblock true
>                                 type hw
>                                 card "PCH"
>                                 device 0
>                                 subdevice 0
>                         }
>
>                         # mplayer2 chooses 1024
>                         # period_size 512 with buffer_size 16384 stops
> crackling in xmame
>                         # 320 breaks flash - https://bbs.archlinux.org/
> viewtopic.php?id=129458
>                         #period_size 512
>                         period_size 1024
>
>                         # 4096 might make sound crackle
>                         # mplayer2 chooses 8192. Half-Life 2 chooses 16384.
>                         # If too large, use CONFIG_SND_HDA_PREALLOC_SIZE=
> 2048
>                         buffer_size 8192
>                 }
>         }
>         capture.pcm "schneeball"
> }
>
> libvirt xml snippets:
>
>     <sound model='ich9'>
>       <address type='pci' domain='0x0000' bus='0x00' slot='0x09'
> function='0x0'/>
>     </sound>
>     ...
>     <qemu:env name='QEMU_AUDIO_DRV' value='alsa'/>
>     <qemu:env name='QEMU_DAC_FIXED_SETTINGS' value='0'/>
>     <qemu:env name='QEMU_DAC_TRY_POLL' value='0'/>
>     <qemu:env name='QEMU_DAC_TIMER_PERIOD' value='0'/>
>     <qemu:env name='QEMU_DAC_BUFFER_SIZE' value='0'/>
>     <qemu:env name='QEMU_DAC_PERIOD_SIZE' value='0'/>
>     <qemu:env name='QEMU_DAC_DEV' value='dmixed'/>
>
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