Re: [PATCH v2 4/4] coco/tdx-host: Initialize trusted DMA on all capable IOMMUs

[Baolu Lu <baolu.lu@linux.intel.com>]

On 10/16/2026 1:37 AM, Jason Gunthorpe wrote:
> On Thu, Oct 15, 2026 at 04:31:31PM +0800, Xu Yilun wrote:
> 
>> An alternative could be, bringing up one IOMMU when an associated
>> device requests link encryption. Then a no-tdx-support iommu fails the
>> encryption request naturally.
> That is what ARM is planning to do. I think it is reasonable to
> trigger creation of the secure iommu when the TSM is connected to TDX.
> 
> Fail connect if the iommu could not be created.

Yes, agreed. The Intel iommu driver could use per-device interfaces,
just as ARM is planning. There are some differences, though.

ARM's interaction with the iommu driver in viommu_init() appears to
consist of two steps:

  - arm_smmu_get_realm_params(dev, &params): get and validate the
    physical iommu information from the arm-smmu-v3 driver;
  - cca_psmmu_get(&params): activate the physical smmu for Realm use.

Intel differs in two ways:

  - ARM queries the physical iommu information from the iommu driver
    in the viommu_init() path. On Intel, the secure iommu must be set
    up during TSM connect, because it is part of establishing PCIe
    link encryption.

  - The TDX Connect architecture places requirements on the host
    iommu driver while the secure IOMMU is being set up. For example,
    the host iommu driver must block the enhanced command (ECMD)
    interface while the TDH.IOMMU.SETUP SEAMCALL runs, and it must
    reserve the upper half of the domain ID space. These steps have
    to happen atomically under the iommu driver's own locks. As a
    result, the Intel iommu driver needs to merge the two steps above
    into one.

So the Intel iommu driver will probably end up with interfaces like:

  - dmar_tdxcs_iommu_init(struct device *dev, ...);
  - dmar_tdxcs_iommu_release(struct device *dev, ...);

These would be called from the connect and disconnect callbacks of
the TDX link ops:

static struct pci_tsm_ops tdx_tsm_link_ops = {
         .owner = THIS_MODULE,
         .probe = tdx_tsm_link_probe,
         .remove = tdx_tsm_link_remove,
         .connect = tdx_tsm_link_connect,
         .disconnect = tdx_tsm_link_disconnect,
...
};

Any thoughts?

Thanks,
baolu