KVM/VFIO GVT-g broken on 5.4.0-rc8?

[Alex Williamson]

I was trying to test John Hubbard's latest gup series[1] with GVT-g,
but I'm unable to run a Windows 10 VM with a i915-GVTg_V4_2 on
af42d3466bdc ("Linux 5.4-rc8").  The host kernel prints:

[   64.402671] gvt: len is not valid:  len=11  valid_len=3
[   64.407932] gvt: vgpu 2: MI_OP_2F handler error
[   64.412502] gvt: vgpu 2: cmd parser error
[   64.416591] 0x0 
[   64.416592] 0x2f 

[   64.421930] gvt: vgpu 2: scan wa ctx error
[   64.426088] GVT Internal error  for the guest
[   64.430486] Now vgpu 2 will enter failsafe mode.
[   64.435179] gvt: vgpu 2: failed to submit desc 0
[   64.439853] gvt: vgpu 2: fail submit workload on ring 0
[   64.445125] gvt: vgpu 2: fail to emulate MMIO write 00002230 len 4
[  103.522117] gvt: len is not valid:  len=11  valid_len=3
[  103.527419] gvt: vgpu 2: MI_OP_2F handler error
[  103.532004] gvt: vgpu 2: cmd parser error
[  103.536062] 0x0 
[  103.536063] 0x2f 

[  103.541342] gvt: vgpu 2: scan wa ctx error
[  103.545466] GVT Internal error  for the guest
[  103.549863] Now vgpu 2 will enter failsafe mode.
[  103.554502] gvt: vgpu 2: failed to submit desc 0
[  103.559156] gvt: vgpu 2: fail submit workload on ring 0
[  103.564388] gvt: vgpu 2: fail to emulate MMIO write 00002230 len 4

And the VM reboots before reaching the desktop, possibly with a BSOD.
The same VM configuration works on host kernel 5.3.11-200.fc30.x86_64.
Device manager reports HD Graphics 5500 driver version 20.19.15.4624,
which was installed by Windows update.  Thanks,

Alex

[1]https://lore.kernel.org/linux-rdma/20191119081643.1866232-1-jhubbard@nvidia.com/T/#t