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

[Baolu Lu]

On 10/16/2026 10:58 PM, Jason Gunthorpe 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...
> That's going to be a bit of a PITA, since the DIDs can be in use when
> the tdx module is start potentially.
>
> I suppose you can start out by just failing if this has happened.
>
> But as soon as reasonably possible once the system opts into a TDX
> operation the VT-d drivers should stop using the upper did space.
>
> Otherwise you will have to implement live DID renaming, which is a
> PITA in an iommu driver.

Yes, agreed.

Thanks,
baolu