[vfio-users] AMD Ryzen Nested Page Table Performance Oddities

[Nick Sarnie]

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 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
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