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    SubjectRe: [PATCH 20/22] kvm: mmu: NX largepage recovery for TDP MMU
    On Fri, Sep 25, 2020 at 02:23:00PM -0700, Ben Gardon wrote:
    > +/*
    > + * Clear non-leaf SPTEs and free the page tables they point to, if those SPTEs
    > + * exist in order to allow execute access on a region that would otherwise be
    > + * mapped as a large page.
    > + */
    > +void kvm_tdp_mmu_recover_nx_lpages(struct kvm *kvm)
    > +{
    > + struct kvm_mmu_page *sp;
    > + bool flush;
    > + int rcu_idx;
    > + unsigned int ratio;
    > + ulong to_zap;
    > + u64 old_spte;
    > +
    > + rcu_idx = srcu_read_lock(&kvm->srcu);
    > + spin_lock(&kvm->mmu_lock);
    > +
    > + ratio = READ_ONCE(nx_huge_pages_recovery_ratio);
    > + to_zap = ratio ? DIV_ROUND_UP(kvm->stat.nx_lpage_splits, ratio) : 0;

    This is broken, and possibly related to Paolo's INIT_LIST_HEAD issue. The TDP
    MMU never increments nx_lpage_splits, it instead has its own counter,
    tdp_mmu_lpage_disallowed_page_count. Unless I'm missing something, to_zap is
    guaranteed to be zero and thus this is completely untested.

    I don't see any reason for a separate tdp_mmu_lpage_disallowed_page_count,
    a single VM can't have both a legacy MMU and a TDP MMU, so it's not like there
    will be collisions with other code incrementing nx_lpage_splits. And the TDP
    MMU should be updating stats anyways.

    > +
    > + while (to_zap &&
    > + !list_empty(&kvm->arch.tdp_mmu_lpage_disallowed_pages)) {
    > + /*
    > + * We use a separate list instead of just using active_mmu_pages
    > + * because the number of lpage_disallowed pages is expected to
    > + * be relatively small compared to the total.
    > + */
    > + sp = list_first_entry(&kvm->arch.tdp_mmu_lpage_disallowed_pages,
    > + struct kvm_mmu_page,
    > + lpage_disallowed_link);
    > +
    > + old_spte = *sp->parent_sptep;
    > + *sp->parent_sptep = 0;
    > +
    > + list_del(&sp->lpage_disallowed_link);
    > + kvm->arch.tdp_mmu_lpage_disallowed_page_count--;
    > +
    > + handle_changed_spte(kvm, kvm_mmu_page_as_id(sp), sp->gfn,
    > + old_spte, 0, sp->role.level + 1);
    > +
    > + flush = true;
    > +
    > + if (!--to_zap || need_resched() ||
    > + spin_needbreak(&kvm->mmu_lock)) {
    > + flush = false;
    > + kvm_flush_remote_tlbs(kvm);
    > + if (to_zap)
    > + cond_resched_lock(&kvm->mmu_lock);
    > + }
    > + }
    > +
    > + if (flush)
    > + kvm_flush_remote_tlbs(kvm);
    > +
    > + spin_unlock(&kvm->mmu_lock);
    > + srcu_read_unlock(&kvm->srcu, rcu_idx);
    > +}
    > +
    > diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
    > index 2ecb047211a6d..45ea2d44545db 100644
    > --- a/arch/x86/kvm/mmu/tdp_mmu.h
    > +++ b/arch/x86/kvm/mmu/tdp_mmu.h
    > @@ -43,4 +43,6 @@ void kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
    >
    > bool kvm_tdp_mmu_write_protect_gfn(struct kvm *kvm,
    > struct kvm_memory_slot *slot, gfn_t gfn);
    > +
    > +void kvm_tdp_mmu_recover_nx_lpages(struct kvm *kvm);
    > #endif /* __KVM_X86_MMU_TDP_MMU_H */
    > --
    > 2.28.0.709.gb0816b6eb0-goog
    >

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