** Description changed:
Title:
Regression: PCI commit 7a126c1b6cfa ("Enable ACS after configuring IOMMU for
OF platforms") merges IOMMU groups on AMD x86, breaks VFIO passthrough — needs
revert 817daf163312 backported
Package: linux (Ubuntu) — noble, 6.8 kernel
Is this a regression? Yes.
Summary
Upstream commit 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF
platforms") was backported into Ubuntu's noble 6.8 kernel in 6.8.0-130.130 (as
part of syncing to upstream stable v6.12.75; see changelog entry for LP:
#2150809). This commit moves pci_enable_acs() into pci_dma_configure(), which
runs on all platforms, not just OF/Device-Tree ones. On AMD x86 systems this
changes ACS evaluation timing relative to IOMMU group assignment, causing
devices that previously sat in isolated IOMMU groups to merge into one —
breaking VFIO PCI passthrough.
This is a known upstream regression, already fixed via revert commit
817daf163312 ("PCI: Revert 'Enable ACS after configuring IOMMU for OF
platforms'"), bisected and confirmed on AMD-Vi hardware. See:
https://forum.level1techs.com/t/iommu-group-regression-in-linux-kernel-6-12-75/247479
The revert is absent from Ubuntu's noble 6.8 changelogs from 6.8.0-130.130
through 6.8.0-134.134.
Impact
VFIO PCI passthrough fails with:
vfio 0000:0b:00.0: group 2 is not viable
Please ensure all devices within the iommu_group are bound to their vfio bus
driver.
Reproduction (my system)
6.8.0-124-generic (pre-regression): two AMD 1022:1453 GPP bridges, each with
a downstream NVIDIA GP107 GPU, sit in separate IOMMU groups (16 and 17).
6.8.0-134-generic (regressed): the same two bridges and both GPUs merge into
a single IOMMU group.
Both kernels use identical cmdline (amd_iommu iommu=pt) and identical
vfio-pci binding via /etc/modprobe.d/vfio.conf (subsystem-ID matched).
System info
CPU/chipset: AMD Family 17h (Zen), AMD 400-series chipset
GPUs: 2x NVIDIA GP107 [GTX 1050 Ti] (10de:1c82) + audio functions (10de:0fb9)
Working: 6.8.0-124-generic (6.8.0-124.124)
Broken: 6.8.0-134-generic (6.8.0-134.134)
Requested fix
Backport 817daf163312 into the noble 6.8 stable branch.
ProblemType: Bug
DistroRelease: Ubuntu 24.04
Package: linux-image-6.8.0-134-generic 6.8.0-134.134
ProcVersionSignature: Ubuntu 6.8.0-134.134-generic 6.8.12
Uname: Linux 6.8.0-134-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.28.1-0ubuntu3.8
Architecture: amd64
AudioDevicesInUse:
- USER PID ACCESS COMMAND
- /dev/snd/controlC1: macro 2576 F.... wireplumber
- /dev/snd/controlC0: macro 2576 F.... wireplumber
- /dev/snd/seq: macro 2574 F.... pipewire
+ USER PID ACCESS COMMAND
+ /dev/snd/controlC1: macro 2576 F.... wireplumber
+ /dev/snd/controlC0: macro 2576 F.... wireplumber
+ /dev/snd/seq: macro 2574 F.... pipewire
CRDA: N/A
CasperMD5CheckResult: unknown
CurrentDesktop: KDE
Date: Wed Jul 1 17:29:33 2026
InstallationDate: Installed on 2018-10-20 (2811 days ago)
InstallationMedia: Kubuntu 18.10 "Cosmic Cuttlefish" - Release amd64
(20181017.2)
MachineType: System manufacturer System Product Name
ProcFB: 0 nvidia-drmdrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-6.8.0-134-generic
root=UUID=6eb14018-7030-43e9-bfbd-634e324040e2 ro quiet splash amd_iommu=on
iommu=pt vt.handoff=7
PulseList: Error: command ['pacmd', 'list'] failed with exit code 1: No
PulseAudio daemon running, or not running as session daemon.
RelatedPackageVersions:
- linux-restricted-modules-6.8.0-134-generic N/A
- linux-backports-modules-6.8.0-134-generic N/A
- linux-firmware 20240318.git3b128b60-0ubuntu2.27
+ linux-restricted-modules-6.8.0-134-generic N/A
+ linux-backports-modules-6.8.0-134-generic N/A
+ linux-firmware 20240318.git3b128b60-0ubuntu2.27
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 09/27/2018
dmi.bios.release: 5.13
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 1001
dmi.board.asset.tag: Default string
dmi.board.name: ROG CROSSHAIR VII HERO (WI-FI)
dmi.board.vendor: ASUSTeK COMPUTER INC.
dmi.board.version: Rev 1.xx
dmi.chassis.asset.tag: Default string
dmi.chassis.type: 3
dmi.chassis.vendor: Default string
dmi.chassis.version: Default string
dmi.modalias:
dmi:bvnAmericanMegatrendsInc.:bvr1001:bd09/27/2018:br5.13:svnSystemmanufacturer:pnSystemProductName:pvrSystemVersion:rvnASUSTeKCOMPUTERINC.:rnROGCROSSHAIRVIIHERO(WI-FI):rvrRev1.xx:cvnDefaultstring:ct3:cvrDefaultstring:skuSKU:
dmi.product.family: To be filled by O.E.M.
dmi.product.name: System Product Name
dmi.product.sku: SKU
dmi.product.version: System Version
dmi.sys.vendor: System manufacturer
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https://bugs.launchpad.net/bugs/2158934
Title:
Regression related to IOMMU grouping
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