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SubjectRe: [PATCH] KVM: x86/MMU: Zap non-leaf SPTEs when disabling dirty logging
On Thu, May 26, 2022 at 8:52 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 5/26/22 16:30, Peter Xu wrote:
> > On Thu, May 26, 2022 at 02:01:43PM +0200, Paolo Bonzini wrote:
> >> On 5/26/22 01:09, Ben Gardon wrote:
> >>> + WARN_ON(max_mapping_level < iter.level);
> >>> +
> >>> + /*
> >>> + * If this page is already mapped at the highest
> >>> + * viable level, there's nothing more to do.
> >>> + */
> >>> + if (max_mapping_level == iter.level)
> >>> + continue;
> >>> +
> >>> + /*
> >>> + * The page can be remapped at a higher level, so step
> >>> + * up to zap the parent SPTE.
> >>> + */
> >>> + while (max_mapping_level > iter.level)
> >>> + tdp_iter_step_up(&iter);
> >>> +
> >>> /* Note, a successful atomic zap also does a remote TLB flush. */
> >>> - if (tdp_mmu_zap_spte_atomic(kvm, &iter))
> >>> - goto retry;
> >>> + tdp_mmu_zap_spte_atomic(kvm, &iter);
> >>> +
> >>
> >> Can you make this a sparate function (for example
> >> tdp_mmu_zap_collapsible_spte_atomic)? Otherwise looks great!
> >
> > There could be a tiny downside of using a helper in that it'll hide the
> > step-up of the iterator, which might not be as obvious as keeping it in the
> > loop?
>
> That's true, my reasoning is that zapping at a higher level can only be
> done by first moving the iterator up. Maybe
> tdp_mmu_zap_at_level_atomic() is a better Though, I can very well apply
> this patch as is.

I'd be inclined to apply the patch as-is for a couple reasons:
1. As Peter said, hiding the step up could be confusing.
2. If we want to try the in-place promotion, we'll have to dismantle
that helper again anyway or else have a bunch of duplicate code.

>
> Paolo
>

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