[Bug 196409] New: kvm_amd nested pagetable gpu passthrough performance oddities

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https://bugzilla.kernel.org/show_bug.cgi?id=196409

            Bug ID: 196409
           Summary: kvm_amd nested pagetable gpu passthrough performance
                    oddities
           Product: Virtualization
           Version: unspecified
    Kernel Version: 4.10.8-1
          Hardware: x86-64
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: high
          Priority: P1
         Component: kvm
          Assignee: virtualization_kvm@xxxxxxxxxxxxxxxxxxxx
          Reporter: efeu@xxxxxxxxxx
        Regression: No

The hardware I was testing with:
AMD Ryzen R7 1700
Gigabyte GA-AX370-Gaming 5
different GPUs
Windows10 x64 Guest

But this bug is reproducable on AMD FX series too.

While community is discussing for a while about this bug, I haven't found it
here. Based on our discussion here:
http://www.spinics.net/lists/kvm/msg149446.html

While npt is enabled a passed through GPU is giving much less performance as
expected. Here some test results the community already did:

First Heaven benchmark with ultra settings on 1920x1080:

- DirectX 11:
  - npt=0: 87.0 fps
  - npt=1: 78.4 fps (10% drop)
- DirectX 9:
  - npt=0: 100.0 fps
  - npt=1: 66.4 fps (33% drop)
- OpenGL:
  - npt=0: 82.5 fps
  - npt=1: 35.2 fps (58% drop)

Heaven Benchmark again, this time with low settings on 1280x720:

- DirectX 11:
  - npt=0: 182.5 fps
  - npt=1: 140.1 fps (25% drop)
- DirectX 9:
  - npt=0: 169.2 fps
  - npt=1: 74.1 fps (56% drop)
- OpenGL:
  - npt=0: 202.8 fps
  - npt=1: 45.0 fps (78% drop)

PerformanceTest 9.0 3d benchmark:

- DirectX 9:
  - npt=0: 157 fps
  - npt=1: 13 fps (92% drop)
- DirectX 10:
  - npt=0: 220 fps
  - npt=1: 212 fps (4% drop)
- DirectX 11:
  - npt=0: 234 fps
  - npt=1: 140 fps (40% drop)
- DirectX 12:
  - npt=0: 88 fps (scored 35 because of the penalized FPS of not being
able to run at 4k)
  - npt=1: 4.5 fps (scored 1, 95% drop)
- GPU Compute:
  - Mandel:
    - npt=0: ~= 2000 fps
    - npt=1: ~= 2000 fps
  - Bitonic Sort:
    - npt=0: ~= 153583696.0 elements/sec
    - npt=1: ~= 106233376.0 elements/sec (31% drop)
  - QJulia4D:
    - npt=0: ~= 1000 fps
    - npt=1: ~= 1000 fps
  - OpenCL:
    - npt=0: ~= 750 fps
    - npt=1: ~= 220 fps
Some more data from 3DMark benchmarks:

Time Spy(DirectX 12):
- Graphics test 1:
  - npt=0: 37.65 FPS
  - npt=1: 24.22 FPS (36% drop)
- Graphics test 2:
  - npt=0: 33.05 FPS
  - npt=1: 29.65 FPS (10% drop)
- CPU test:
  - npt=0: 17.35 FPS
  - npt=1: 12.03 FPS (31% drop)

Fire Strike(DirectX 11):
- Graphics test 1:
  - npt=0: 80.56 FPS
  - npt=1: 41.89 FPS (49% drop)
- Graphics test 2:
  - npt=0: 70.64 FPS
  - npt=1: 60.75 FPS (14% drop)
- Physics test:
  - npt=0: 50.14 FPS
  - npt=1: 5.78 FPS (89% drop)
- Combined test:
  - npt=0: 32.83 FPS
  - npt=1: 17.70 FPS (47% drop)

Sky Diver(DirectX 11):
- Graphics test 1:
  - npt=0: 248.81 FPS
  - npt=1: 173.63 FPS (31% drop)
- Graphics test 2:
  - npt=0: 250.49 FPS
  - npt=1: 124.84 FPS (51% drop)
- Physics test:
  - 8 threads:
    - npt=0: 140.93 FPS
    - npt=1: 119.08 FPS (15% drop)
  - 24 threads:
    - npt=0: 110.22 FPS
    - npt=1: 74.55 FPS (33% drop)
  - 48 threads:
    - npt=1: 71.56 FPS
    - npt=1: 45.93 FPS (36% drop)
  - 96 threads:
    - npt=0: 41.04 FPS
    - npt=1: 24.81 FPS (40% drop)
- Combined test:
  - npt=0: 75.65 FPS
  - npt=1: 50.45 FPS (33% drop)

I compared the performance with XEN and found out there is no performance
impact, so the bug should be in the nested pagetable implementation in kvm_amd
module and not a hardware related issue in AMD-Vi.

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