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    SubjectRe: [RFC PATCH v2 4/6] KVM: x86: Introduce fault type to indicate kvm page fault is private
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    On Thu, Jun 22, 2023, isaku.yamahata@intel.com wrote:
    > diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
    > index 7f9ec1e5b136..0ec0b927a391 100644
    > --- a/arch/x86/kvm/mmu/mmu_internal.h
    > +++ b/arch/x86/kvm/mmu/mmu_internal.h
    > @@ -188,6 +188,13 @@ static inline bool is_nx_huge_page_enabled(struct kvm *kvm)
    > return READ_ONCE(nx_huge_pages) && !kvm->arch.disable_nx_huge_pages;
    > }
    >
    > +enum kvm_fault_type {
    > + KVM_FAULT_MEM_ATTR,
    > + KVM_FAULT_SHARED,
    > + KVM_FAULT_SHARED_ALWAYS,
    > + KVM_FAULT_PRIVATE,

    This is silly. Just use AMD's error code bit, i.e. PFERR_GUEST_ENC_MASK as per
    the SNP series.

    Bit 34 (ENC): Set to 1 if the guest’s effective C-bit was 1, 0 otherwise.

    Just because Intel's ucode is too crusty to support error codes larger than 16
    bits doesn't mean KVM can't utilize the bits :-) KVM already translates to AMD's
    error codes for other things, e.g.

    error_code |= (exit_qualification & EPT_VIOLATION_GVA_TRANSLATED) != 0 ?
    PFERR_GUEST_FINAL_MASK : PFERR_GUEST_PAGE_MASK;

    For TDX, handle_ept_violation() can do something like:

    if (is_tdx(vcpu->kvm))
    error_code |= (gpa & shared) ? 0 : PFERR_GUEST_ENC_MASK;
    else if (kvm_mem_is_private(vcpu->kvm, gpa_to_gfn(gpa)))
    error_code |= PFERR_GUEST_ENC_MASK;

    And that's not even taking into account that TDX might have a separate entry point,
    i.e. the "is_tdx()" check can probably be avoided.

    As for optimizing kvm_mem_is_private() to avoid unnecessary xarray lookups, that
    can and should be done separately, e.g.

    static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
    {
    return IS_ENABLED(CONFIG_KVM_PRIVATE_MEM) &&
    kvm_guest_has_private_mem(kvm) &&
    kvm_get_memory_attributes(kvm, gfn) & KVM_MEMORY_ATTRIBUTE_PRIVATE;
    }

    where x86's implementation of kvm_guest_has_private_mem() can be

    #define kvm_guest_has_private_mem(kvm) (!!(kvm)->vm_type)

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