Arch Linux

HoltFrak

Member

Registered: 2022-01-16

Posts: 7

Clarification on using ACS patch to break up IOMMU groups

Hi all,
I'm trying to set up GPU passthrough as per the Arch wiki on the subject for the purpose of passing the GPU to a Windows VM. My system has two GPUs that are unfortunately in the same IOMMU group. I cannot put the second GPU into another PCIe slot as there is no room in my case. Here is the offending IOMMU group:

 IOMMU Group 1:
        00:01.0 PCI bridge [0604]: Intel Corporation 6th-10th Gen Core Processor PCIe Controller (x16) [8086:1901] (rev 07)
        00:01.1 PCI bridge [0604]: Intel Corporation Xeon E3-1200 v5/E3-1500 v5/6th Gen Core Processor PCIe Controller (x8) [8086:1905] (rev 07)
        01:00.0 VGA compatible controller [0300]: NVIDIA Corporation GP104 [GeForce GTX 1080] [10de:1b80] (rev a1)
        01:00.1 Audio device [0403]: NVIDIA Corporation GP104 High Definition Audio Controller [10de:10f0] (rev a1)
        02:00.0 VGA compatible controller [0300]: NVIDIA Corporation GM204 [GeForce GTX 980] [10de:13c0] (rev a1)
        02:00.1 Audio device [0403]: NVIDIA Corporation GM204 High Definition Audio Controller [10de:0fbb] (rev a1)

Using the "pcie_acs_override=downstream,multifunction" command strikes me as the nuclear option as far as ACS patching is concerned, and I would like to avoid it since I don't need to isolate every single device into its own group. I found a video on ACS patching here (relevant section at 18:20) that mentions using the override command to specify a single PCI bridge ID to (I believe) break up a singular IOMMU group. However, in the video a capture card is isolated, not a GPU. If I were to use "pcie_acs_override=id:8086:1905," would that break up IOMMU group 1 in such a way that both the video and audio controller for a GPU would be included in a new group? Or would I have to specify several IDs into one "pcie_acs_override" command to get a bridge, video and audio device in a new group?

Last edited by HoltFrak (2022-01-23 19:26:28)