Hi

Host: Arch running kernel 4.20.3 / Qemu 3.1
Guest: Ubuntu 18.04.1 (tried Ubuntu 18.10 also) with any kernel after 4.15
Driver: Happens with current stable, git from Padoka PPA and amdgpu-pro 18.50

Issue: Vega 64 passed through to guest causes X to hang on boot using 100% CPU
for one of the passed through cores for the Xorg process. X never starts with
the stopping point being 'LoadModule: "dri2"'. I cannot see any relevant errors
in Xorg.log or the KRB - though it's easily reproducible. The system is still
alive and can be ssh'd into.

Adding the nomodeset kernel option allows the guest to boot to GUI (albeit
without acceleration).

Details: With Qemu 3.0 having more than ~12GB RAM assigned to the guest causes
this behaviour. With Qemu 3.1 the amount of RAM is irrelevant - the hang always
occurs.

Believe this is related to one of the GPU reset patches added to the 4.16
kernel as the guest boots fine with Qemu 3.0/3.1 and guest kernel 4.15 (tested
up to 16GB guest RAM).