[vfio-users] Radeon 5770 Passthrough works in Chipset 440FX but not in Q35

[Laszlo Ersek]

On 06/02/16 20:57, Zir Blazer wrote:

> Where I had absolutely no success with Q35 is trying to install the
> Radeon Drivers, since without them, the VM is worthless for gaming. I
> tested the three versions available for the Radeon 5xxx series for
> Windows 10 (Catalyst 15.7, 15.7.1, which I have in my working Xen
> install, and Crimson Edition 16.2.1 Beta). Everytime that I get to the
> Display Driver install stage, be it either when installing the full
> Catalyst package, or by using Update Driver in the Device Manager, the
> screen turns black, after a few seconds the Monitor enters Standby, and
> the VM doesn't provide life signals any longer, so I have to forcefully
> close it. There is no useful dmesg message besides some kvm [1968]:
> vcpu0 unhandled rdmsr: 0x641 (Something like that, don't have the
> precise dmesg output at hand), but seems to be unrelated to this.

You are wrong about this. When the guest reads a model-specific register
that KVM doesn't handle, *and* you don't pass the ignore_msrs=1
parameter to the "kvm" module, then KVM will inject a general protection
fault in response to the RDMSR instruction. The guest likely doesn't
expect this fault, and then anything can happen at all.

According to the Intel SDM, 0x641 is MSR_PP1_ENERGY_STATUS, and it
belongs to "PLATFORM SPECIFIC POWER MANAGEMENT SUPPORT". I don't know
why the Radeon driver (apparently) reads that MSR on Q35, and not on
i440fx, but it is certainly consistent with a crash.

https://www.redhat.com/archives/vfio-users/2016-May/msg00099.html

Laszlo