linux-kernel - Re: [PATCH] KVM: SVM: Disable TDP MMU when running on Hyper-V

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Date:   Thu, 13 Apr 2023 12:09:44 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tianyu Lan <ltykernel@...il.com>,
        Michael Kelley <mikelley@...rosoft.com>
Subject: Re: [PATCH] KVM: SVM: Disable TDP MMU when running on Hyper-V

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).