[ANNOUNCE] PUCK Notes - 2024.01.17 - TDP MMU for IOMMU

[Sean Christopherson]

Recording and slides:

  https://drive.google.com/corp/drive/folders/1sSr_8FE5KjjGGnpX7_QlHAX3QGoRnck7?resourcekey=0-UB_vbXfpY4Dezo9xI_-6iA

Key Takeways:

 - Having KVM notify (or install PTEs in) the IOMMU page tables for _all_ PTEs
   created by KVM may not be necessary to achieve the desired performance, e.g.
   proactively mapping in the IOMMU may only be necessary when swapping in
   memory for oversubscribed VMs.

 - Synchronously notifying/installing could be a net negative for guest
   performance, e.g. could add significant latency in KVM's page fault path if
   a PTE operation necessitates an IOMMU TLB invalidation.

 - Despite hardware vendors' intentions/claims, CPU and IOMMU page table entries
   aren't 100% interchangeable.  E.g. even on Intel, where the formats are
   compatible, it's still possible to create EPT PTEs (CPU) that are not usable
   in the IOMMU.

 - Given the above, having KVM manage and/or notify IOMMU Page tables would be
   a premature optimization.

 - Recommended next step is to explore using heterogeneous memory management
   (HMM) to manage IOMMU page tables and coordinate with mmu_notifiers, and see
   if HMM can be optimized to meet the performance goals without involving KVM.