Regression in 6.17.13-1/2-pve: DMAR faults with VFIO when intel_iommu=on iommu=pt (works on 6.17.9-1-pve)

[Jonathan50543]

Hello,

I would like to report what appears to be a kernel regression affecting VFIO passthrough.

Summary​

On my system VFIO passthrough works correctly with kernel 6.17.9-1-pve, but after upgrading to 6.17.13-1-pve or 6.17.13-2-pve, DMAR faults start appearing after the VM runs for some time.

Downgrading back to 6.17.9-1-pve makes the issue disappear completely, suggesting a regression introduced between these kernel versions.

Kernel parameters​

The host kernel is booted with:

intel_iommu=on iommu=pt

Symptoms​

After the VM has been running for a while, the host begins logging repeated DMAR faults such as:

[ 8955.704107] DMAR: DRHD: handling fault status reg 3
[ 8955.704115] DMAR: [DMA Write NO_PASID] Request device [00:0d.0] fault addr 0x64d87000 [fault reason 0x05] PTE Write access is not set
[ 8955.704146] DMAR: DRHD: handling fault status reg 3
[ 8955.704150] DMAR: [DMA Write NO_PASID] Request device [00:0d.0] fault addr 0x64d8a000 [fault reason 0x05] PTE Write access is not set
[ 8955.704194] DMAR: DRHD: handling fault status reg 3
[ 8955.704197] DMAR: [DMA Write NO_PASID] Request device [00:0d.0] fault addr 0x64d8c000 [fault reason 0x05] PTE Write access is not set
[ 8961.574179] dmar_fault: 2057 callbacks suppressed
[ 8961.574185] DMAR: DRHD: handling fault status reg 3
[ 8961.574190] DMAR: [DMA Read NO_PASID] Request device [00:0d.0] fault addr 0x408b5000 [fault reason 0x06] PTE Read access is not set
[ 8963.079753] DMAR: DRHD: handling fault status reg 3
[ 8963.079761] DMAR: [DMA Read NO_PASID] Request device [05:00.4] fault addr 0x40a65000 [fault reason 0x06] PTE Read access is not set
[ 8963.080493] DMAR: DRHD: handling fault status reg 3
[ 8963.080497] DMAR: [DMA Read NO_PASID] Request device [05:00.4] fault addr 0x40a65000 [fault reason 0x06] PTE Read access is not set
[ 9037.575995] dmar_fault: 29 callbacks suppressed
[ 9037.575999] DMAR: DRHD: handling fault status reg 3
[ 9037.576003] DMAR: [DMA Read NO_PASID] Request device [05:00.4] fault addr 0x40bd0000 [fault reason 0x06] PTE Read access is not set

Two devices attached to the VM appear in the fault logs.

Hardware​

Platform: Intel N305 (Alder Lake-N)

Devices passed through via VFIO:

00:0d.0 USB controller [0c03]: Intel Corporation Alder Lake-N Thunderbolt 4 USB Controller [8086:464e]
05:00.4 Ethernet controller [0200]: Mellanox Technologies MT27800 Family [ConnectX-5 Virtual Function] [15b3:1018]

VM configuration (relevant parts)​

BIOS:

SeaBIOS

CPU configuration:

cpu: host,flags=-nested-virt;-virt-ssbd;-amd-ssbd;-amd-no-ssb;-pdpe1gb;-hv-tlbflush;-hv-evmcs

Passthrough devices:

hostpci1: 0000:00:0d,pcie=1,rombar=0
hostpci2: 0000:05:00.4,pcie=1,rombar=0

Guest​

The guest is running a 6.12 kernel.

Regression confirmation​

• 6.17.9-1-pve → no DMAR faults, passthrough stable
• 6.17.13-1-pve → DMAR faults appear
• 6.17.13-2-pve → same issue
• Downgrading back to 6.17.9-1-pve → issue disappears

Additional info​

To avoid exposing unrelated hardware details, I have only included the relevant log excerpts and device information above.
If more targeted debugging information is required, I can provide it.

Thanks.