[REGRESSION] Too-low frequency limit for AMD GPU PCI-passed-through to Windows VM

[James Turner]

> Are you ever loading the amdgpu driver in your tests?

Yes, although I'm binding the `vfio-pci` driver to the AMD GPU's PCI
devices via the kernel command line. (See my initial email.) My
understanding is that `vfio-pci` is supposed to keep other drivers, such
as `amdgpu`, from interacting with the GPU, although that's clearly not
what's happening.

I've been testing with `amdgpu` included in the `MODULES` list in
`/etc/mkinitcpio.conf` (which Arch Linux uses to generate the
initramfs). However, I ran some more tests today (results below), this
time without `i915` or `amdgpu` in the `MODULES` list. The `amdgpu`
kernel module still gets loaded. (I think udev loads it automatically?)

Your comment gave me the idea to blacklist the `amdgpu` kernel module.
That does serve as a workaround on my machine – it fixes the behavior
for f9b7f3703ff9 ("drm/amdgpu/acpi: make ATPX/ATCS structures global (v2)")
and for the current Arch Linux prebuilt kernel (5.16.2-arch1-1). That's
an acceptable workaround for my machine only because the separate GPU
used by the host is an Intel integrated GPU. That workaround wouldn't
work well for someone with two AMD GPUs.


# New test results

The following tests are set up the same way as in my initial email,
with the following exceptions:

- I've updated libvirt to 1:8.0.0-1.

- I've removed `i915` and `amdgpu` from the `MODULES` list in
  `/etc/mkinitcpio.conf`.

For all three of these tests, `lspci` said the following:

% lspci -nnk -d 1002:6981
01:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] Lexa XT [Radeon PRO WX 3200] [1002:6981]
	Subsystem: Dell Device [1028:0926]
	Kernel driver in use: vfio-pci
	Kernel modules: amdgpu

% lspci -nnk -d 1002:aae0
01:00.1 Audio device [0403]: Advanced Micro Devices, Inc. [AMD/ATI] Baffin HDMI/DP Audio [Radeon RX 550 640SP / RX 560/560X] [1002:aae0]
	Subsystem: Dell Device [1028:0926]
	Kernel driver in use: vfio-pci
	Kernel modules: snd_hda_intel


## Version f1688bd69ec4 ("drm/amd/amdgpu:save psp ring wptr to avoid attack")

This is the commit immediately preceding the one which introduced the issue.

% sudo dmesg | grep -i amdgpu
[   15.840160] [drm] amdgpu kernel modesetting enabled.
[   15.840884] amdgpu: CRAT table not found
[   15.840885] amdgpu: Virtual CRAT table created for CPU
[   15.840893] amdgpu: Topology: Add CPU node

% lsmod | grep amdgpu
amdgpu               7450624  0
gpu_sched              49152  1 amdgpu
drm_ttm_helper         16384  1 amdgpu
ttm                    77824  2 amdgpu,drm_ttm_helper
i2c_algo_bit           16384  2 amdgpu,i915
drm_kms_helper        303104  2 amdgpu,i915
drm                   581632  11 gpu_sched,drm_kms_helper,amdgpu,drm_ttm_helper,i915,ttm

The passed-through GPU worked properly in the VM.


## Version f9b7f3703ff9 ("drm/amdgpu/acpi: make ATPX/ATCS structures global (v2)")

This is the commit which introduced the issue.

% sudo dmesg | grep -i amdgpu
[   15.319023] [drm] amdgpu kernel modesetting enabled.
[   15.329468] amdgpu: CRAT table not found
[   15.329470] amdgpu: Virtual CRAT table created for CPU
[   15.329482] amdgpu: Topology: Add CPU node

% lsmod | grep amdgpu
amdgpu               7450624  0
gpu_sched              49152  1 amdgpu
drm_ttm_helper         16384  1 amdgpu
ttm                    77824  2 amdgpu,drm_ttm_helper
i2c_algo_bit           16384  2 amdgpu,i915
drm_kms_helper        303104  2 amdgpu,i915
drm                   581632  11 gpu_sched,drm_kms_helper,amdgpu,drm_ttm_helper,i915,ttm

The passed-through GPU did not run above 501 MHz in the VM.


## Blacklisted `amdgpu`, version f9b7f3703ff9 ("drm/amdgpu/acpi: make ATPX/ATCS structures global (v2)")

For this test, I added `module_blacklist=amdgpu` to kernel command line
to blacklist the `amdgpu` module.

% sudo dmesg | grep -i amdgpu
[   14.591576] Module amdgpu is blacklisted

% lsmod | grep amdgpu

The passed-through GPU worked properly in the VM.


James