linux-kernel - Re: [PATCH] KVM: x86/MMU: Zap non-leaf SPTEs when disabling dirty logging

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Date:   Thu, 26 May 2022 09:00:29 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Peter Xu <peterx@...hat.com>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        David Dunn <daviddunn@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Junaid Shahid <junaids@...gle.com>
Subject: Re: [PATCH] KVM: x86/MMU: Zap non-leaf SPTEs when disabling dirty logging

On Thu, May 26, 2022 at 8:52 AM Paolo Bonzini <pbonzini@...hat.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
>