Re: [PATCH 20/22] kvm: mmu: NX largepage recovery for TDP MMU

[Ben Gardon <bgardon@google.com>]

On Wed, Sep 30, 2020 at 11:16 AM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> 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.

Good catch, I should write some NX reclaim selftests.

>
> 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.

A VM actually can have both the legacy MMU and TDP MMU, by design. The
legacy MMU handles nested. Eventually I'd like the TDP MMU to be
responsible for building nested shadow TDP tables, but haven't
implemented it.

>
> > +
> > +     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
> >