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    Subject[PATCH v4 2/3] KVM: x86/mmu: Avoid collision with !PRESENT SPTEs in TDP MMU lpage stats
    From
    From: Sean Christopherson <seanjc@google.com>

    Factor in whether or not the old/new SPTEs are shadow-present when
    adjusting the large page stats in the TDP MMU. A modified MMIO SPTE can
    toggle the page size bit, as bit 7 is used to store the MMIO generation,
    i.e. is_large_pte() can get a false positive when called on a MMIO SPTE.
    Ditto for nuking SPTEs with REMOVED_SPTE, which sets bit 7 in its magic
    value.

    Opportunistically move the logic below the check to verify at least one
    of the old/new SPTEs is shadow present.

    Use is/was_leaf even though is/was_present would suffice. The code
    generation is roughly equivalent since all flags need to be computed
    prior to the code in question, and using the *_leaf flags will minimize
    the diff in a future enhancement to account all pages, i.e. will change
    the check to "is_leaf != was_leaf".

    Reviewed-by: David Matlack <dmatlack@google.com>
    Reviewed-by: Ben Gardon <bgardon@google.com>

    Fixes: 1699f65c8b65 ("kvm/x86: Fix 'lpages' kvm stat for TDM MMU")
    Cc: stable@vger.kernel.org
    Signed-off-by: Sean Christopherson <seanjc@google.com>
    Signed-off-by: Mingwei Zhang <mizhang@google.com>
    ---
    arch/x86/kvm/mmu/tdp_mmu.c | 20 +++++++++++++-------
    1 file changed, 13 insertions(+), 7 deletions(-)

    diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
    index 43f12f5d12c0..4b0953fed12e 100644
    --- a/arch/x86/kvm/mmu/tdp_mmu.c
    +++ b/arch/x86/kvm/mmu/tdp_mmu.c
    @@ -413,6 +413,7 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
    bool was_leaf = was_present && is_last_spte(old_spte, level);
    bool is_leaf = is_present && is_last_spte(new_spte, level);
    bool pfn_changed = spte_to_pfn(old_spte) != spte_to_pfn(new_spte);
    + bool was_large, is_large;

    WARN_ON(level > PT64_ROOT_MAX_LEVEL);
    WARN_ON(level < PG_LEVEL_4K);
    @@ -446,13 +447,6 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,

    trace_kvm_tdp_mmu_spte_changed(as_id, gfn, level, old_spte, new_spte);

    - if (is_large_pte(old_spte) != is_large_pte(new_spte)) {
    - if (is_large_pte(old_spte))
    - atomic64_sub(1, (atomic64_t*)&kvm->stat.lpages);
    - else
    - atomic64_add(1, (atomic64_t*)&kvm->stat.lpages);
    - }
    -
    /*
    * The only times a SPTE should be changed from a non-present to
    * non-present state is when an MMIO entry is installed/modified/
    @@ -478,6 +472,18 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
    return;
    }

    + /*
    + * Update large page stats if a large page is being zapped, created, or
    + * is replacing an existing shadow page.
    + */
    + was_large = was_leaf && is_large_pte(old_spte);
    + is_large = is_leaf && is_large_pte(new_spte);
    + if (was_large != is_large) {
    + if (was_large)
    + atomic64_sub(1, (atomic64_t *)&kvm->stat.lpages);
    + else
    + atomic64_add(1, (atomic64_t *)&kvm->stat.lpages);
    + }

    if (was_leaf && is_dirty_spte(old_spte) &&
    (!is_present || !is_dirty_spte(new_spte) || pfn_changed))
    --
    2.32.0.554.ge1b32706d8-goog
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