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    SubjectRe: [PATCH 1/7] KVM: Write protect the updated slot only when we start dirty logging
    On Tue, Dec 25, 2012 at 01:08:40PM +0900, Takuya Yoshikawa wrote:
    > On Mon, 24 Dec 2012 15:27:17 +0200
    > Gleb Natapov <gleb@redhat.com> wrote:
    >
    > > > @@ -6897,7 +6897,14 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
    > > > spin_lock(&kvm->mmu_lock);
    > > > if (nr_mmu_pages)
    > > > kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages);
    > > > - kvm_mmu_slot_remove_write_access(kvm, mem->slot);
    > > > + /*
    > > > + * Write protect all pages for dirty logging.
    > > > + * Existing largepage mappings are destroyed here and new ones will
    > > > + * not be created until the end of the logging.
    > > > + */
    > > > + if ((mem->flags & KVM_MEM_LOG_DIRTY_PAGES) &&
    > > > + !(old.flags & KVM_MEM_LOG_DIRTY_PAGES))
    > > > + kvm_mmu_slot_remove_write_access(kvm, mem->slot);
    > > We should not check old slot flags here or at least check that
    > > old.npages is not zero. Userspace may delete a slot using old flags,
    > > then, if new memslot is created with dirty log enabled, it will not be
    > > protected.
    >
    > The flag, KVM_MEM_LOG_DIRTY_PAGES, is explicitely cleared when we
    > delete a slot to free dirty_bitmap:
    >
    > if (!npages)
    > mem->flags &= ~KVM_MEM_LOG_DIRTY_PAGES;
    >
    > So when old.npages is not zero, the second condition must be true.
    >
    Right. I was looking for where it is cleared and missed that.

    > Other parts are doing "if (!slot->dirty_bitmap)" to see if the slot
    > is in dirty logging mode. If you prefer, we can do the same here.
    >
    Does not matter to me. If you think it will be more consistent do it.

    --
    Gleb.


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