Re: [Patch v2 2/5] KVM: x86/mmu: Optimize SPTE change flow for clear-dirty-log

[Vipin Sharma]

On Mon, Feb 6, 2023 at 2:06 PM Ben Gardon <bgardon@xxxxxxxxxx> wrote:
>
> On Fri, Feb 3, 2023 at 11:28 AM Vipin Sharma <vipinsh@xxxxxxxxxx> wrote:
> > diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
> > index 30a52e5e68de..21046b34f94e 100644
> > --- a/arch/x86/kvm/mmu/tdp_iter.h
> > +++ b/arch/x86/kvm/mmu/tdp_iter.h
> > @@ -121,4 +121,17 @@ void tdp_iter_start(struct tdp_iter *iter, struct kvm_mmu_page *root,
> >  void tdp_iter_next(struct tdp_iter *iter);
> >  void tdp_iter_restart(struct tdp_iter *iter);
> >
> > +static inline u64 kvm_tdp_mmu_clear_spte_bit(struct tdp_iter *iter, u64 mask)
> > +{
> > +       atomic64_t *sptep;
> > +
> > +       if (kvm_tdp_mmu_spte_has_volatile_bits(iter->old_spte, iter->level)) {
> > +               sptep = (atomic64_t *)rcu_dereference(iter->sptep);
> > +               return (u64)atomic64_fetch_and(~mask, sptep);
> > +       }
> > +
> > +       __kvm_tdp_mmu_write_spte(iter->sptep, iter->old_spte & ~mask);
> > +       return iter->old_spte;
> > +}
> > +
>
> If you do end up sending another version of the series and feel like
> breaking up this patch, you could probably split this part out since
> the change to how we set the SPTE and how we handle that change are
> somewhat independent. I like the switch to atomic64_fetch_and though.
> No idea if it's faster, but I would believe it could be.

David explained in his email why it is not independent.

>
> Totally optional, but there's currently no validation on the mask.
> Maybe we're only calling this in one place, but it might be worth
> clarifying the limits (if any) on what bits can be set in the mask. I
> don't think there necessarily need to be limits as of this commit, but
> the handling around this function where it's called here would
> obviously not be sufficient if the mask were -1UL or something.
>

I cannot think of any specific mask to be useful here. Let us keep it
as it is, we can revisit this API if there is a need to add a mask in
future. If someone sends -1UL then it will be on them on how they are
using the API.