On Thu, Apr 13, 2023, Sean Christopherson wrote: > Aha! Idea. There are _at most_ 4 possible roots the TDP MMU can encounter. > 4-level non-SMM, 4-level SMM, 5-level non-SMM, and 5-level SMM. I.e. not keeping > inactive roots on a per-VM basis is just monumentally stupid. One correction: there are 6 possible roots: 1. 4-level !SMM !guest_mode (i.e. not nested) 2. 4-level SMM !guest_mode 3. 5-level !SMM !guest_mode 4. 5-level SMM !guest_mode 5. 4-level !SMM guest_mode 6. 5-level !SMM guest_mode I forgot that KVM still uses the TDP MMU when running L2 if L1 doesn't enable EPT/TDP, i.e. if L1 is using shadow paging for L2. But that really doesn't change anything as each vCPU can already track 4 roots, i.e. userspace can saturate all 6 roots anyways. And in practice, no sane VMM will create a VM with both 4-level and 5-level roots (KVM keys off of guest.MAXPHYADDR for the TDP root level).
