Re: [PATCH v3] iommu/vt-d: Disable IOMMU by default on Lenovo ThinkPad P50s

On 27/07/2026 12:03, Tian, Kevin wrote:
> If it's confirmed to be a hardware issue, it should be (or will be) documented
> as an VT-d erratum. Then a quirk could be added accordingly.
>
> If Lenovo confirms to be a firmware issue, then they should release a new
> firmware version to fix it. It doesn't justify a quirk in iommu driver.
>
> and it is not about a policy "whether this issue should be *treated* as...".
> It needs a root cause first.


Hi Kevin,

I performed additional investigation on the DMA fault.

With intel_iommu enabled:

DMAR: [DMA Read NO_PASID] Request device [0000:00:12.4] fault addr 
0xb7fff000 [fault reason 0x02] Present bit in context entry is clear

However, requester ID 0000:00:12.4 does not appear in PCI enumeration:

- `lspci -nn -s 00:12.4` returns no output
- `lspci -xxxx -s 00:12.0` through `00:12.4` all return no device
- `/sys/bus/pci/devices/` contains no matching entry

I also checked the firmware-provided ACPI DMAR table. It contains scopes 
for 00:02.0 and 00:14.0, but no scope for 00:12.4.

Could this requester ID correspond to an internal platform device, or 
does this suggest a firmware/platform issue?

Thanks.