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    SubjectRe: [PATCH v7 035/102] KVM: x86/mmu: Explicitly check for MMIO spte in fast page fault
    On Thu, Jun 30, 2022 at 11:37:15PM +1200,
    Kai Huang <kai.huang@intel.com> wrote:

    > On Mon, 2022-06-27 at 14:53 -0700, isaku.yamahata@intel.com wrote:
    > > From: Sean Christopherson <sean.j.christopherson@intel.com>
    > >
    > > Explicitly check for an MMIO spte in the fast page fault flow. TDX will
    > > use a not-present entry for MMIO sptes, which can be mistaken for an
    > > access-tracked spte since both have SPTE_SPECIAL_MASK set.
    >
    > SPTE_SPECIAL_MASK has been removed in latest KVM code. The changelog needs
    > update.

    It was renamed to SPTE_TDP_AD_MASK. not removed.


    > In fact, if I understand correctly, I don't think this changelog is correct:

    > The existing code doesn't check is_mmio_spte() because:
    >
    > 1) If MMIO caching is enabled, MMIO fault is always handled in
    > handle_mmio_page_fault() before reaching here;
    >
    > 2) If MMIO caching is disabled, is_shadow_present_pte() always returns false for
    > MMIO spte, and is_mmio_spte() also always return false for MMIO spte, so there's
    > no need check here.
    >
    > "A non-present entry for MMIO spte" doesn't necessarily mean
    > is_shadow_present_pte() will return true for it, and there's no explanation at
    > all that for TDX guest a MMIO spte could reach here and is_shadow_present_pte()
    > returns true for it.

    Although it was needed, I noticed the following commit made this patch
    unnecessary. So I'll drop this patch. Kudos to Sean.

    edea7c4fc215c7ee1cc98363b016ad505cbac9f7
    "KVM: x86/mmu: Use a dedicated bit to track shadow/MMU-present SPTEs"

    --
    Isaku Yamahata <isaku.yamahata@gmail.com>

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