A question of TDP unloading.

[Yu Zhang <yu.c.zhang@linux.intel.com>]

Hi all,

  I'd like to ask a question about kvm_reset_context(): is there any
  reason that we must alway unload TDP root in kvm_mmu_reset_context()?
  
  As you know, KVM MMU needs to track guest paging mode changes, to
  recalculate the mmu roles and reset callback routines(e.g., guest
  page table walker). These are done in kvm_mmu_reset_context(). Also,
  entering SMM, cpuid updates, and restoring L1 VMM's host state will
  trigger kvm_mmu_reset_context() too.
  
  Meanwhile, another job done by kvm_mmu_reset_context() is to unload
  the KVM MMU:
  
  - For shadow & legacy TDP, it means to unload the root shadow/TDP
    page and reconstruct another one in kvm_mmu_reload(), before
    entering guest. Old shadow/TDP pages will probably be reused later,
    after future guest paging mode switches.
  
  - For TDP MMU, it is even more aggressive, all TDP pages will be
    zapped, meaning a whole new TDP page table will be recontrustred,
    with each paging mode change in the guest. I witnessed dozens of
    rebuildings of TDP when booting a Linux guest(besides the ones
    caused by memslots rearrangement).
  
  However, I am wondering, why do we need the unloading, if GPA->HPA
  relationship is not changed? And if this is not a must, could we
  find a way to refactor kvm_mmu_reset_context(), so that unloading
  of TDP root is only performed when necessary(e.g, SMM switches and
  maybe after cpuid updates which may change the level of TDP)? 
  
  I tried to add a parameter in kvm_mmu_reset_context(), to make the
  unloading optional:  

+void kvm_mmu_reset_context(struct kvm_vcpu *vcpu, bool force_tdp_unload)
 {
-       kvm_mmu_unload(vcpu);
+       if (!tdp_enabled || force_tdp_unload)
+               kvm_mmu_unload(vcpu);
+
        kvm_init_mmu(vcpu);
 }

  But this change brings another problem - if we keep the TDP root, the
  role of existing SPs will be obsolete after guest paging mode changes.
  Altough I guess most role flags are irrelevant in TDP, I am not sure
  if this could cause any trouble.
  
  Is there anyone looking at this issue? Or do you have any suggestion?
  Thanks!
  
B.R.
Yu