On Wed, Apr 5, 2017 at 11:32 PM, Nick Sarnie <commendsarnex@xxxxxxxxx> wrote: > Hi all, > > I'm seeing some strange issues regarding the Nested Page Table option > of the kvm-amd module on my Ryzen system. > > First off, having the parameter enabled, as it is by default, clearly > and consistently increases CPU performance inside the VM. I've tested > this with a kernel compile, and it is around 5 times longer with npt > disabled using tmpfs. Passing any PCI device through to the VM does > not affect the CPU performance. > > Now, if I pass through my graphics card, we see some very strange > issues. If npt is disabled, the FPS in 3D applications is between 2 Sorry, I mean 'if npt is enabled'. > and 4 times lower than if npt is disabled. This is a consistent, > cross-VM-platform result. In Windows 10, Dota 2 runs at around 35 FPS > with npt enabled, and around 110 with npt disabled. In Fedora 25, > Heaven benchmark results in an average of 32 FPS with npt enabled, and > 48 with npt disabled. I haven't found a 3D application where there is > not a significant GPU performance hit to enabling npt. > > I would like to try to understand this issue, as anyone wanting to use > a GPU Passthrough config on Ryzen basically is forced to disable npt > and suffer a CPU performance hit. > > I can provide any performance statistics, logs, or test any patches. > > Thank you, > Sarnex