[Qemu-devel] [Bug 1795527] Re: Malformed audio and video output stuttering after upgrade to QEMU 3.0

[tlloss]

Ran through the commits included in the audio code merge, with the
following results:

[commit 280c1e1cdb24d80ecdfcdfc679ccc5e8ed7af45d]
audio/hda: create millisecond timers that handle IO

Audio stream gets progressively more and more corrupted, breaking completely 
between 30'' and 1' after continuous sound start.
No problems playing short sounds.

--

[commit 0a373bb310c1533e24aa5e3edbf206507fb342ea]
audio/hda: turn some dprintfs into trace points

No changes from the previous commit, of course.

--

[commit 8ced0669237b2bbedac3e4ce6fcf7aaaafaae663]
audio/hda: tweak timer adjust logic

First time audio looks really good on this guest; the new code is
working, by the look of things.

--

[commit 4501ee16c76e89e0a2b2beb95f3b93f965997391]
audio/hda: detect output buffer overruns

First time issue presents itself.

--

So, I assume that commit 4501ee16c76e89e0a2b2beb95f3b93f965997391 introduced 
some kind of overrun control which mishandles the buffer, at least in my setup.
>From a quick and ignorant git diff between this and the previous commit, I can 
>see that the new detector could drop the buffer too early, or maybe it 
>misconfigures the st->buft_start property.

These last tests were performed by manually toggling the use-timer
property on from inside the source code; I hope this doesn't invalidate
their outcome, though.

As of now I have no clue on how to patch this thing, since I do not
understand the interactions between the various emulator's components.

-- 
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https://bugs.launchpad.net/bugs/1795527

Title:
  Malformed audio and video output stuttering after upgrade to QEMU 3.0

Status in QEMU:
  New

Bug description:
  My host is an x86_64 Arch Linux OS with a recompiled 4.18.10 hardened
  kernel, running a few KVM guests with varying OSes and configurations
  managed through a Libvirt stack.

  Among these guests I have two Windows 10 VMs with VGA passthrough and
  PulseAudio-backed virtual audio devices.

  After upgrading to QEMU 3.0.0, both of the Win10 guests started
  showing corrupted audio output in the form of unnatural reproduction
  speed and occasional but consistently misplaced audio fragments
  originating from what seems to be a circular buffer wrapping over
  itself (misbehaviour detected by starting some games with known OSTs
  and dialogues: soundtracks sound accelerated and past dialogue lines
  start replaying middle-sentence until the next line starts playing).

  In addition, the video output of the malfunctioning VMs regularly
  stutters roughly twice a second for a fraction of a second (sync'ed
  with the suspected buffer wrapping and especially pronounced during
  not-pre-rendered cutscenes), toghether with mouse freezes that look
  like actual input misses more than simple lack of screen refreshes.

  
  The issue was succesfully reproduced without the managing stack, directly 
with the following command line, on the most capable Windows guest:

   QEMU_AUDIO_DRV=pa
   QEMU_PA_SERVER=127.0.0.1
   /usr/bin/qemu-system-x86_64 -name guest=win10_gms,debug-threads=on \
   -machine pc-i440fx-3.0,accel=kvm,usb=off,vmport=off,dump-guest-core=off \    
                                                                                
                                                       
   -cpu 
host,hv_time,hv_relaxed,hv_vapic,hv_spinlocks=0x1fff,hv_vendor_id=123456789abc,kvm=off
 \          
   -drive 
file=/usr/share/ovmf/x64/OVMF_CODE.fd,if=pflash,format=raw,unit=0,readonly=on \ 
      
   -drive 
file=/var/lib/libvirt/qemu/nvram/win10_gms_VARS.fd,if=pflash,format=raw,unit=1 \
   -m 5120 \                                                                    
          
   -realtime mlock=off \
   -smp 3,sockets=1,cores=3,threads=1 \
   -uuid 39b56ee2-6bae-4009-9108-7be26d5d63ac \
   -display none \                             
   -no-user-config \
   -nodefaults \    
   -rtc base=localtime,driftfix=slew \                                          
                                                                                
                                                       
   -global kvm-pit.lost_tick_policy=delay \                                     
                             
   -no-hpet \                              
   -no-shutdown \
   -global PIIX4_PM.disable_s3=1 \
   -global PIIX4_PM.disable_s4=1 \
   -boot strict=on \              
   -device ich9-usb-ehci1,id=usb,bus=pci.0,addr=0x4.0x7 \
   -device 
ich9-usb-uhci1,masterbus=usb.0,firstport=0,bus=pci.0,multifunction=on,addr=0x4 \
   -device ich9-usb-uhci2,masterbus=usb.0,firstport=2,bus=pci.0,addr=0x4.0x1 \  
           
   -device ich9-usb-uhci3,masterbus=usb.0,firstport=4,bus=pci.0,addr=0x4.0x2 \
   -device ahci,id=sata0,bus=pci.0,addr=0x9 \                                 
   -drive 
file=/dev/vms/win10_gaming,format=raw,if=none,id=drive-virtio-disk0,cache=none,aio=native
 \
   -device 
virtio-blk-pci,scsi=off,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1,write-cache=on
 \
   -drive 
file=/dev/sr0,format=raw,if=none,id=drive-sata0-0-0,media=cdrom,readonly=on \   
                                 
   -device ide-cd,bus=sata0.0,drive=drive-sata0-0-0,id=sata0-0-0 \              
       
   -device intel-hda,id=sound0,bus=pci.0,addr=0x3 \                             
                                                                                
                                                       
   -device hda-duplex,id=sound0-codec0,bus=sound0.0,cad=0 \                     
                        
   -device usb-host,hostbus=2,hostaddr=3,id=hostdev0,bus=usb.0,port=1 \
   -device vfio-pci,host=01:00.0,id=hostdev1,bus=pci.0,addr=0x6 \      
   -device vfio-pci,host=01:00.1,id=hostdev2,bus=pci.0,addr=0x7 \
   -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x8 \   
   -sandbox 
on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny \
   -msg timestamp=on

  
  By "purposedly misconfiguring" the codepaths and replacing "pc-i440fx-3.0" 
with "pc-i440fx-2.11" (basically reverting the config changes I needed to do in 
order to update the domain definitions), the stuttering seems to disappear (or 
at least becomes negligible) and the audio output, despite becoming incredibly 
distorted, is consistent in every other way, with in-order dialogues and 
(perceived) correct tempo.

  
  In order to exclude eventual misconfigurations in the host's audio processing 
pipeline, I proceeded to update the domain definition's codepath of another 
guest running Ubuntu 18.04 with a completely different hardware configuration 
(no video card passthrough and no PulseAudio backconnection, just a plain 
emulated VirtIO display and Spice audio device).

  The audio issue presented itself again in the form of slightly sped up audio 
playback from Internet videos interleaved with occasional "quenches" of playing 
speed.
  Stutters are difficult to detect because of the poor refresh rate of the 
emulated VGA adapter, but I wouldn't be surprised to find them here too 
(actually, I *think* I sensed them, but I'm not sure enough to assess their 
existence).

  Once again, by reverting to the old 2.11 directive everything is back
  to normal.


  Given the fact that no official upgrade directives regarding required 
sampling rate, period or sheduling adjustments were stated or handed-out to 
administrators, I decided to report this behaviour as a bug.
  I hope this is the appropriate channel and that I didn't annoy anyone (this 
is my first proper bug report, please forgive me for any innaccuracy).

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