[vfio-users] RT priorty on vCPU threads destroys everything?

[Michael Slade]

This may not strictly be VFIO-related but it is latency-related. Feel 
free to suggest another place to post.

I have a VM which has 4 of the host's 8 cores, and has multiple PCI 
devices passed through - GPU, audio, USB and SATA.

If I give the vCPU threads RT priority (via libvirt XML), and then run 
`stress -c 4` on the VM, the host would freeze.

If I dedicate the 4 cores to the VM via isolcpus= and CPU pinning, then 
the host is okay but there are still other issues, e.g. the guest's 
audio glitches like crazy.

Everything is fine (well mostly fine) if I refrain from setting RT 
priority on the vCPU threads.

What's happening here? Is this a scheduling bug?

Host and guest are running kernel 5.4.91, and qemu is stable-5.0.