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

[Izik Eidus]

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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