[Qemu-devel] PCI Passthrough + AMD + NPT

[geoff@hostfission.com]

Hi All,

I have started to dig into why ntp seems to slow down graphics 
performance on
AMD systems using PCI passthrough and figured I would report what I have 
so far
discovered. I have noted the primary point of failure seems to be 
specifically
with PhysX. This is why people only see a slow down in certain games, 
not
everything uses PhysX.

Using FluidMark[1] the problem is immediately obvious, showing extremely 
low
FPS on light/medium workloads with ntp enabled, and extreme fluididy and 
high
FPS with ntp disabled.

Switching nVidia to use CPU makes no difference to the performance when 
ntp
is enabled, which seems to indicate that PhysX is falling back to CPU 
due to
a failure of some kind to initialize.

With ntp turned off, and nVidia set to use the CPU for PhysX I see an
identical performance drop off in FluidMark as I see when ntp is 
enabled, this
would seem to confirm this suspicion.

Since other features such as APIC is only available if ntp is enabled, 
it
could be something down stream of ntp that is getting disabled as a
consequence of turning off ntp. It might be interesting to see if we can 
get
some diagnostics information out of PhysX to see what if any error or 
debugging
information it might provide when it falls back to CPU.

1: 
http://www.geeks3d.com/20130308/fluidmark-1-5-1-physx-benchmark-fluid-sph-simulation-opengl-download/

Kind Regards,
Geoffrey McRae