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SubjectRe: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking
On 6/22/2011 1:43 PM, Avi Kivity wrote:
> On 06/21/2011 04:32 PM, Nai Xia wrote:
>> Introduced kvm_mmu_notifier_test_and_clear_dirty(),
>> kvm_mmu_notifier_dirty_update()
>> and their mmu_notifier interfaces to support KSM dirty bit tracking,
>> which brings
>> significant performance gain in volatile pages scanning in KSM.
>> Currently, kvm_mmu_notifier_dirty_update() returns 0 if and only if
>> intel EPT is
>> enabled to indicate that the dirty bits of underlying sptes are not
>> updated by
>> hardware.
>>
>
>
> Can you quantify the performance gains?
>
>> +int kvm_test_and_clear_dirty_rmapp(struct kvm *kvm, unsigned long
>> *rmapp,
>> + unsigned long data)
>> +{
>> + u64 *spte;
>> + int dirty = 0;
>> +
>> + if (!shadow_dirty_mask) {
>> + WARN(1, "KVM: do NOT try to test dirty bit in EPT\n");
>> + goto out;
>> + }
>> +
>> + spte = rmap_next(kvm, rmapp, NULL);
>> + while (spte) {
>> + int _dirty;
>> + u64 _spte = *spte;
>> + BUG_ON(!(_spte& PT_PRESENT_MASK));
>> + _dirty = _spte& PT_DIRTY_MASK;
>> + if (_dirty) {
>> + dirty = 1;
>> + clear_bit(PT_DIRTY_SHIFT, (unsigned long *)spte);
>> + }
>
> Racy. Also, needs a tlb flush eventually.

Hi, one of the issues is that the whole point of this patch is not do
tlb flush eventually,
But I see your point, because other users will not expect such behavior,
so maybe there is need into a parameter
flush_tlb=?, or add another mmu notifier call?



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