linux-kernel - Re: [PATCH 17/22] kvm: mmu: Support dirty logging for the TDP MMU

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Message-ID: <CANgfPd8itkAnPqr=PfFn3Jf1O_NbY90AEQBKDQ8OR14CagDOzg@mail.gmail.com>
Date:   Thu, 8 Oct 2020 11:27:37 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 17/22] kvm: mmu: Support dirty logging for the TDP MMU

On Fri, Sep 25, 2020 at 6:04 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 25/09/20 23:22, Ben Gardon wrote:
> >                               start_level, KVM_MAX_HUGEPAGE_LEVEL, false);
> > +     if (kvm->arch.tdp_mmu_enabled)
> > +             flush = kvm_tdp_mmu_wrprot_slot(kvm, memslot, false) || flush;
> >       spin_unlock(&kvm->mmu_lock);
> >
>
> In fact you can just pass down the end-level KVM_MAX_HUGEPAGE_LEVEL or
> PGLEVEL_4K here to kvm_tdp_mmu_wrprot_slot and from there to
> wrprot_gfn_range.

That makes sense. My only worry there is the added complexity of error
handling values besides PG_LEVEL_2M and PG_LEVEL_4K. Since there are
only two callers, I don't think that will be too much of a problem
though. I don't think KVM_MAX_HUGEPAGE_LEVEL would actually be a good
value to pass in as I don't think that would write protect 2M
mappings. KVM_MAX_HUGEPAGE_LEVEL is defined as PG_LEVEL_1G, or 3.

>
> >
> > +             /*
> > +              * Take a reference on the root so that it cannot be freed if
> > +              * this thread releases the MMU lock and yields in this loop.
> > +              */
> > +             get_tdp_mmu_root(kvm, root);
> > +
> > +             spte_set = wrprot_gfn_range(kvm, root, slot->base_gfn,
> > +                             slot->base_gfn + slot->npages, skip_4k) ||
> > +                        spte_set;
> > +
> > +             put_tdp_mmu_root(kvm, root);
>
>
> Generalyl using "|=" is the more common idiom in mmu.c.

I changed to this in response to some feedback on the RFC, about
mixing bitwise ops and bools, but I like the |= syntax more as well.

>
> > +static bool clear_dirty_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
> > +                        gfn_t start, gfn_t end)
> > ...
> > +             __handle_changed_spte(kvm, as_id, iter.gfn, iter.old_spte,
> > +                                   new_spte, iter.level);
> > +             handle_changed_spte_acc_track(iter.old_spte, new_spte,
> > +                                           iter.level);
>
> Is it worth not calling handle_changed_spte?  handle_changed_spte_dlog
> obviously will never fire but duplicating the code is a bit ugly.
>
> I guess this patch is the first one that really gives the "feeling" of
> what the data structures look like.  The main difference with the shadow
> MMU is that you have the tdp_iter instead of the callback-based code of
> slot_handle_level_range, but otherwise it's not hard to follow one if
> you know the other.  Reorganizing the code so that mmu.c is little more
> than a wrapper around the two will help as well in this respect.
>
> Paolo
>