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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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