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    SubjectRe: [PATCH 1/2] KVM: Block memslot updates across range_start() and range_end()
    On Tue, Jul 27, 2021, Paolo Bonzini wrote:
    > @@ -764,8 +769,9 @@ static inline struct kvm_memslots *__kvm_memslots(struct kvm *kvm, int as_id)
    > {
    > as_id = array_index_nospec(as_id, KVM_ADDRESS_SPACE_NUM);
    > return srcu_dereference_check(kvm->memslots[as_id], &kvm->srcu,
    > - lockdep_is_held(&kvm->slots_lock) ||
    > - !refcount_read(&kvm->users_count));
    > + lockdep_is_held(&kvm->slots_lock) ||
    > + READ_ONCE(kvm->mn_active_invalidate_count) ||

    Hmm, I'm not sure we should add mn_active_invalidate_count as an exception to
    holding kvm->srcu. It made sense in original (flawed) approach because the
    exception was a locked_is_held() check, i.e. it was verifying the the current
    task holds the lock. With mn_active_invalidate_count, this only verifies that
    there's an invalidation in-progress, it doesn't verify that this task/CPU is the
    one doing the invalidation.

    Since __kvm_handle_hva_range() takes SRCU for read, maybe it's best omit this?

    > + !refcount_read(&kvm->users_count));
    > }
    >
    > static inline struct kvm_memslots *kvm_memslots(struct kvm *kvm)

    ...

    > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
    > index 5cc79373827f..c64a7de60846 100644
    > --- a/virt/kvm/kvm_main.c
    > +++ b/virt/kvm/kvm_main.c
    > @@ -605,10 +605,8 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
    >
    > /*
    > * .change_pte() must be surrounded by .invalidate_range_{start,end}(),

    Nit, the comma can be switch to a period. The next patch starts a new sentence,
    so it would be correct even in the long term.

    > - * and so always runs with an elevated notifier count. This obviates
    > - * the need to bump the sequence count.
    > */
    > - WARN_ON_ONCE(!kvm->mmu_notifier_count);
    > + WARN_ON_ONCE(!READ_ONCE(kvm->mn_active_invalidate_count));
    >
    > kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn);
    > }

    Nits aside,

    Reviewed-by: Sean Christopherson <seanjc@google.com>

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