Poor windows VFIO performance, GPU stalls (bisected)

[Geoffrey McRae]

Hi All,

The commit 22540ca3d00d2990a4148a13b92209c3dc5422db causes a Windows KVM 
guest running under QEMU with a VFIO passthrough GPU to randomly stall 
when using the GPU leading to the guest assuming that the driver has 
hung. Reverting this commit resolves the problem.

The host system is configured with the following kernel arguments which 
may be related:
  isolcpus=0-5,24-29,6-11,30-35 rcu_nocbs=0-5,24-29,6-11,30-35

The system is an AMD Threadripper 2970WX on a Gigabyte x399 AORUS Gaming 
7 board.
It has two GPUs each being passed through to two separate KVM guests, 
one is an AMD Radeon 7 in a Linux guest, the other is a GeForce 1080Ti 
in a Windows guest.
The cores used for these two guests are isolated from the host for 
performance reasons.

Any insight as to why this is occurring would be appreciated. If you 
need any more information or would like to test patches please let me 
know.

Kind Regards,
Geoffrey McRae
HostFission

https://hostfission.com