Re: [PATCH v2 2/3] KVM: x86/mmu: Avoid collision with !PRESENT SPTEs in TDP MMU lpage stats

[Ben Gardon <bgardon@google.com>]

On Mon, Jul 26, 2021 at 10:54 AM Mingwei Zhang <mizhang@google.com> wrote:
>
> 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".
>
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Mingwei Zhang <mizhang@google.com>

Reviewed-by: Ben Gardon <bgardon@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 caac4ddb46df..cba2ab5db2a0 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))
> --
> 2.32.0.432.gabb21c7263-goog
>