Public bug reported: 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 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 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 ** Affects: linux (Ubuntu) Importance: Undecided Status: New ** Tags: amd64 apport-bug noble -- You received this bug notification because you are subscribed to linux in Ubuntu. Matching subscriptions: Bgg, Bmail, Nb https://bugs.launchpad.net/bugs/2158934 Title: Regression related to IOMMU grouping Status in linux package in Ubuntu: New Bug description: 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 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 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 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2158934/+subscriptions
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