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    SubjectRe: [PATCH 08/16] KVM: x86/mmu: Bug the VM if kvm_zap_gfn_range() is called for TDX
    On Thu, May 16, 2024 at 12:13:44AM +0000,
    "Edgecombe, Rick P" <rick.p.edgecombe@intel.com> wrote:

    > On Thu, 2024-05-16 at 11:38 +1200, Huang, Kai wrote:
    > > On 16/05/2024 11:14 am, Edgecombe, Rick P wrote:
    > > > On Thu, 2024-05-16 at 10:17 +1200, Huang, Kai wrote:
    > > > > > TDX has several aspects related to the TDP MMU.
    > > > > > 1) Based on the faulting GPA, determine which KVM page table to walk.
    > > > > >        (private-vs-shared)
    > > > > > 2) Need to call TDX SEAMCALL to operate on Secure-EPT instead of direct
    > > > > > memory
    > > > > >        load/store.  TDP MMU needs hooks for it.
    > > > > > 3) The tables must be zapped from the leaf. not the root or the middle.
    > > > > >
    > > > > > For 1) and 2), what about something like this?  TDX backend code will
    > > > > > set
    > > > > > kvm->arch.has_mirrored_pt = true; I think we will use
    > > > > > kvm_gfn_shared_mask()
    > > > > > only
    > > > > > for address conversion (shared<->private).
    > > >
    > > > 1 and 2 are not the same as "mirrored" though. You could have a design that
    > > > mirrors half of the EPT and doesn't track it with separate roots. In fact, 1
    > > > might be just a KVM design choice, even for TDX.
    > >
    > > I am not sure whether I understand this correctly.  If they are not
    > > tracked with separate roots, it means they use the same page table (root).
    >
    > There are three roots, right? Shared, private and mirrored. Shared and mirrored
    > don't have to be different roots, but it makes some operations arguably easier
    > to have it that way.

    Do you have something like KVM_X86_SW_PROTECTED_VM with mirrored PT in mind?
    or TDX thing?



    > > So IIUC what you said is to support "mirror PT" at any sub-tree of the
    > > page table?
    > >
    > > That will only complicate things.  I don't think we should consider
    > > this.  In reality, we only have TDX and SEV-SNP.  We should have a
    > > simple solution to cover both of them.
    >
    > Look at "bool is_private" in kvm_tdp_mmu_map(). Do you see how it switches
    > between different roots in the iterator? That is one use.
    >
    > The second use is to decide whether to call out to the x86_ops. It happens via
    > the role bit in the sp, which is copied from the parent sp role. The root's bit
    > is set originally via a kvm_gfn_shared_mask() check.
    >
    > BTW, the role bit is the thing I'm wondering if we really need, because we have
    > shared_mask. While the shared_mask is used for lots of things today, we need
    > still need it for masking GPAs. Where as the role bit is only needed to know if
    > a SP is for private (which we can tell from the GPA).

    I started the discussion at [1] for it.

    [1] https://lore.kernel.org/kvm/20240516001530.GG168153@ls.amr.corp.intel.com/
    --
    Isaku Yamahata <isaku.yamahata@intel.com>

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