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

[Baolu Lu]

On 10/16/2026 3:17 PM, Tian, Kevin wrote:
> >    - 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.
> Didn't get this. Both ECMD and DID reservation are TDX specific things.
> How are they related to the two steps in the ARM sides and why calls
> for merging?
>
> and why are they must be done atomically? they are conditions to
> execute TDH.IOMMU.SETUP, but it is perfectly fine to reserve DID
> at an early point then block ECMD later...

You are right, "atomically" was too strong. Only the ECMD block has to
bracket TDH.IOMMU.SETUP. So I plan to restructure it as follows:

- dmar_tdxcs_init_prepare(dev, &params): resolve the device's DRHD and
  take a reference on its secure-mode state. For the first user,
  validate the preconditions (TE/QI/SM enabled), reserve the upper half
  of the DID space (failing with -EBUSY if any of it is in use), block
  ECMD and return what tdx-host needs (register base, NUMA node, etc).
  If the unit is already in Secure TDX mode, just report that so
  tdx-host skips TDH.IOMMU.SETUP.

- dmar_tdxcs_init_done(dev, result): unblock ECMD, then either mark the
  unit as being in Secure TDX mode or roll back (including the DID
  reservation), based on the SEAMCALL result. It must be called
  whenever init_prepare() returns success.

- dmar_tdxcs_release(dev): drop the reference; the last user releases
  the DID reservation and clears the Secure TDX mode state after
  tdx-host issues TDH.IOMMU.CLEAR.

These are called from tdx_tsm_link_connect()/disconnect(). Any failure,
whether a precondition, DID reservation or the SEAMCALL itself, aborts
connect with the corresponding error.

Thanks,
baolu