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

[tlloss <1795527@bugs.launchpad.net>]

UPDATE 2 (STUTTERING ELIMINATED, AUDIO ISSUES _STILL_ PRESENT)

I think I've tracked down the source of the stuttering that affected my machine, and it doesn't seem to be QEMU-related.
I'm going to write something about it here anyway, waiting to report it to other, more appropriate channels, hoping that it could help someone else out there.


QUICK FIX:
toggle the `kvm=off` flag off, perform a boot/shutdown cycle, toggle it back on, boot again.


POSSIBLE EXPLANATION:
During the week of panicking trial&error troubleshooting between the discovery of the audio issue and this report's filling, I let Windows perform an update.

It downloaded and installed KB4100347.

Now, by the looks of things this update naively checks the MSR for hipervisor signatures and, not finding any because of the KVM hidden state flag, it carries on with its duties of putting in place whatever microcode-powered mitigations for Spectre V2 it is instructed to set up.
Being the CPU really a virtual one, the process, unsurprisingly, fails, and the OS is left with some sort of software gimmick which tries to compensate for those unsupported functions.

To my surprise, it seems that the MSR check is implemented as a
persistent trigger and, as soon as it detects an hypervisor signature,
it permanently removes the mitigation routines, leaving a smoothly
running VM.

I've performed 3 reboots since I toggled the flag off and on again, and
the performance are consistent.

Obviously, this explanation is just a wild guess, but I find it
believable enough and in line with my experience.


As I stated in the title, though, *the audio corruption is still well
present* under QEMU 3.0, and eliminating the stuttering didn't help,
leaving me with the original and most important problem.

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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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