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

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Date:   Sun, 13 Mar 2022 14:33:41 -0400
From:   James Turner <linuxkernel.foss@...rc-none.turner.link>
To:     Thorsten Leemhuis <regressions@...mhuis.info>
Cc:     Alex Deucher <alexdeucher@...il.com>,
        "Lazar, Lijo" <lijo.lazar@....com>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        "regressions@...ts.linux.dev" <regressions@...ts.linux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        "amd-gfx@...ts.freedesktop.org" <amd-gfx@...ts.freedesktop.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Koenig, Christian" <Christian.Koenig@....com>
Subject: Re: [REGRESSION] Too-low frequency limit for AMD GPU
 PCI-passed-through to Windows VM

Hi all,

I've confirmed that changing the `amdgpu_atif_pci_probe_handle` function
to do nothing does make the GPU work properly in the VM. I started with
f9b7f3703ff9 ("drm/amdgpu/acpi: make ATPX/ATCS structures global (v2)")
and changed the function implementation to:

static bool amdgpu_atif_pci_probe_handle(struct pci_dev *pdev)
{
	DRM_DEBUG_DRIVER("Entered amdgpu_atif_pci_probe_handle");
	return false;
}

With that change, the GPU works properly in the VM.

I'm not sure where to go from here. This issue isn't much of a concern
for me anymore, since blacklisting `amdgpu` works for my machine. At
this point, my understanding is that the root problem needs to be fixed
in AMD's Windows GPU driver or Dell's firmware, not the Linux kernel. If
any of the AMD developers on this thread would like to forward it to the
AMD Windows driver team, I'd be happy to work with AMD to fix the issue
properly.

I've added a mention of this issue and workaround to the [Arch Wiki][1]
to make it more discoverable. If anyone has a better place to document
this, please let me know.

Thank you all for your help on this.

[1]: https://wiki.archlinux.org/title/PCI_passthrough_via_OVMF#Too-low_frequency_limit_for_AMD_GPU_passed-through_to_virtual_machine

James