[vfio-users] Why does the audio from VM crackle?

[Milos Kaurin]

Yes, this keeps coming up now and again on this list.

My solution was:

1. Buy a dedicated PCIE sound card
2. Pass it through to the guest
3. Route audio: mobo soundcard > guest soundcard

In essence, the always-on guest would handle audio, but I would still have
crackles (rarely), especially from the web browsers (host & guest). Games
not so much.

To fix crackles, I did the following:
1. In the kernel cmdline: Use hugepages, 16x1GB for the guest (16 for the
host, 32 total)
2. In the kernel cmdline: Use cpuisol (6 cores for the gues, 2 for the host)
3. In the kernel cmdline: Use nohz_full and rcu_nocbs for the guest cores
4. In the virt XML: Use cputune, hugepages, cpu mode=host-passthrough with
the correct topology
5. In the virt XML: iothreadpin and emulatorpin for the 2 host CPUs
6. In the virt XML: Use virtio wherever possible (net, block devices)

I think I covered everything. If anyone else has any other tips, please let
me know.



Appenix


My /etc/default/grub:

GRUB_CMDLINE_LINUX="rhgb quiet intel_iommu=on iommu=pt kvm.ignore_msrs=1
rd.driver.pre=vfio-pci hugepagesz=1G default_hugepagesz=1G hugepages=16
nohz_full=1,2,3,5,6,7 rcu_nocbs=1,2,3,5,6,7 isolcpus=1,2,3,5,6,7"



My XML:

<domain type='kvm'>
  <name>Win10</name>
  <uuid>4bb72e5b-c886-425f-8280-b69755ebf054</uuid>
  <memory unit='KiB'>16777216</memory>
  <currentMemory unit='KiB'>16777216</currentMemory>
  <memoryBacking>
    <hugepages/>
  </memoryBacking>
  <vcpu placement='static'>6</vcpu>
  <iothreads>2</iothreads>
  <cputune>
    <vcpupin vcpu='0' cpuset='1'/>
    <vcpupin vcpu='1' cpuset='5'/>
    <vcpupin vcpu='2' cpuset='2'/>
    <vcpupin vcpu='3' cpuset='6'/>
    <vcpupin vcpu='4' cpuset='3'/>
    <vcpupin vcpu='5' cpuset='7'/>
    <emulatorpin cpuset='0,4'/>
    <iothreadpin iothread='1' cpuset='0'/>
    <iothreadpin iothread='2' cpuset='4'/>
  </cputune>
  <os>
    <type arch='x86_64' machine='pc-i440fx-2.4'>hvm</type>
    <loader readonly='yes'
type='pflash'>/usr/share/edk2.git/ovmf-x64/OVMF_CODE-pure-efi.fd</loader>
    <nvram>/var/lib/libvirt/qemu/nvram/Win10_VARS.fd</nvram>
  </os>
  <features>
    <acpi/>
    <apic/>
    <kvm>
      <hidden state='on'/>
    </kvm>
    <vmport state='off'/>
  </features>
  <cpu mode='host-passthrough'>
    <topology sockets='1' cores='3' threads='2'/>
  </cpu>
  <clock offset='localtime'>
    <timer name='rtc' tickpolicy='catchup'/>
    <timer name='pit' tickpolicy='delay'/>
    <timer name='hpet' present='no'/>
  </clock>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <pm>
    <suspend-to-mem enabled='no'/>
    <suspend-to-disk enabled='no'/>
  </pm>
  <devices>
    <emulator>/usr/bin/qemu-kvm</emulator>
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none' io='native'/>
      <source dev='/dev/sdb'/>
      <target dev='vda' bus='virtio'/>
      <boot order='2'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x09'
function='0x0'/>
    </disk>
    <controller type='pci' index='0' model='pci-root'/>
    <controller type='scsi' index='0' model='virtio-scsi'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05'
function='0x0'/>
    </controller>
    <controller type='usb' index='0' model='none'/>
    <interface type='bridge'>
      <mac address='52:54:00:3e:c8:03'/>
      <source bridge='br0'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03'
function='0x0'/>
    </interface>
    <hostdev mode='subsystem' type='pci' managed='yes'>
      <source>
        <address domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
      </source>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0a'
function='0x0'/>
    </hostdev>
    <hostdev mode='subsystem' type='pci' managed='yes'>
      <source>
        <address domain='0x0000' bus='0x01' slot='0x00' function='0x1'/>
      </source>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0b'
function='0x0'/>
    </hostdev>
    <hostdev mode='subsystem' type='pci' managed='yes'>
      <source>
        <address domain='0x0000' bus='0x05' slot='0x00' function='0x0'/>
      </source>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02'
function='0x0'/>
    </hostdev>
    <hostdev mode='subsystem' type='pci' managed='yes'>
      <source>
        <address domain='0x0000' bus='0x06' slot='0x00' function='0x0'/>
      </source>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04'
function='0x0'/>
    </hostdev>
    <memballoon model='none'/>
  </devices>
</domain>



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