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    SubjectRe: [PATCH 02/22] kvm: mmu: Introduce tdp_iter
    On Thu, Oct 01, 2020 at 01:34:53AM +0200, Paolo Bonzini wrote:
    > On 01/10/20 01:20, Eric van Tassell wrote:
    > >>
    > >> +int is_shadow_present_pte(u64 pte)
    > >>   {
    > >>       return (pte != 0) && !is_mmio_spte(pte);
    > > From <Figure 28-1: Formats of EPTP and EPT Paging-Structure Entries" of
    > > the manual I don't have at my fingertips right now, I believe you should
    > > only check the low 3 bits(mask = 0x7). Since the upper bits are ignored,
    > > might that not mean they're not guaranteed to be 0?
    >
    > No, this a property of the KVM MMU (and how it builds its PTEs) rather
    > than the hardware present check.

    Ya, I found out the hard way that "present" in is_shadow_present_pte() really
    means "valid", or "may be present". The most notable case is EPT without A/D
    bits (I think this is the only case where a valid SPTE can be fully not-present
    in hardware). Accessed tracking will clear all RWX bits to make the EPT entry
    not-present, but from KVM's perspective it's treated as valid/present because
    it can be made present in hardware without taking the MMU lock.

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