(In reply to Andrey Grodzovsky from comment #6)
> So with Arch Linux kernel it happens only during S3 but with
> amd-staging-drm-next it happens once you start X ?

Yes. I know it sounds strange, but it's currently 100% reproducible to me:

Booting linux-4.17.2-ARCH with amdgpu.vm_update_mode=0:
 X11 starts fine, but system crashes after minutes of firefox browsing

Booting linux-4.17.2-ARCH with amdgpu.vm_update_mode=3:
 X11 starts fine, system does not crash (for at least hours of use)
 but crashes as above if resumed from S3 sleep

Booting linux compiled from amd-staging-drm-next, as of commit
527d6e839a0e52b744fd092453544e4f58977334 from yesterday, with
amdgpu.vm_update_mode=0:
 X11 starts fine, but system crashes after minutes of firefox browsing

Booting linux compiled from amd-staging-drm-next, as of commit
527d6e839a0e52b744fd092453544e4f58977334 from yesterday, with
amdgpu.vm_update_mode=3:
 X11 does not start, crashes immediately with the same above pasted kernel BUG
message and backtrace

So something with CPU-based vm_update_mode is broken, but in a different way
than the SDMA-based method.

I will change the subject of this report to reflect that this crash is not
necessarily S3-resume-related.