Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking

[Izik Eidus <izik.eidus@ravellosystems.com>]

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?