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

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Message-ID: <CALzav=dv2MhoZ1BLqJWmmJv=H6vRaRUEcAJPydjrzJf1wdYEOA@mail.gmail.com>
Date:   Thu, 13 Apr 2023 13:21:44 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>,
        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 at 12:10 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> 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).

Why do we create a new root for guest_mode=1 if L1 disables EPT/NPT?