KVM: x86/mmu: Avoid collision with !PRESENT SPTEs in TDP MMU lpage stats

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>
Message-Id: <20210803044607.599629-3-mizhang@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index fb1b2dc7a6d129b2476f1e6a5e226e1dd6cc64a5..85f69558b490944e264aff6b630bb4e3f2dac3e1 100644 (file)
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -412,6 +412,7 @@ 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);
@@ -445,13 +446,6 @@ 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/
@@ -477,6 +471,18 @@ 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))