Re: [PATCH rfcv3 15/21] intel_iommu: Bind/unbind guest page table to host

[Nicolin Chen]

On Mon, Jun 16, 2025 at 08:15:11AM +0000, Duan, Zhenzhong wrote:
> >IIUIC, the guest kernel cmdline can switch the mode between the
> >stage1 (nesting) and stage2 (legacy/emulated VT-d), right?
> 
> Right. E.g., kexec from "intel_iommu=on,sm_on" to "intel_iommu=on,sm_off",
> Then first kernel will run in scalable mode and use stage1(nesting) and
> second kernel will run in legacy mode and use stage2.

In scalable mode, guest kernel has a stage1 (nested) domain and
host kernel has a stage2 (nesting parent) domain. In this case,
the VFIO container IOAS could be the system AS corresponding to
the kernel-managed stage2 domain.

In legacy mode, guest kernel has a stage2 (normal) domain while
host kernel has a stage2 (shadow) domain? In this case, the VFIO
container IOAS should be the iommu AS corresponding to the kernel
guest-level stage2 domain (or should it be shadow)?

The ARM model that Shameer is proposing only allows a nested SMMU
when such a legacy mode is off. This simplifies a lot of things.
But the difficulty of the VT-d model is that it has to rely on a
guest bootcmd during runtime..

Nicolin