linux-kernel - Re: [PATCH 18/22] kvm: mmu: Support disabling dirty logging for the tdp MMU

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Message-ID: <CANgfPd_dQ19sZz2wzSfz7-RzdbQrfP6cYJLpSYbyNyQW6Uf09Q@mail.gmail.com>
Date:   Wed, 7 Oct 2020 09:30:24 -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 18/22] kvm: mmu: Support disabling dirty logging for the
 tdp MMU

On Fri, Sep 25, 2020 at 6:09 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 25/09/20 23:22, Ben Gardon wrote:
> > +     for_each_tdp_pte_root(iter, root, start, end) {
> > +             if (!is_shadow_present_pte(iter.old_spte) ||
> > +                 is_last_spte(iter.old_spte, iter.level))
> > +                     continue;
> > +
>
> I'm starting to wonder if another iterator like
> for_each_tdp_leaf_pte_root would be clearer, since this idiom repeats
> itself quite often.  The tdp_iter_next_leaf function would be easily
> implemented as
>
>         while (likely(iter->valid) &&
>                (!is_shadow_present_pte(iter.old_spte) ||
>                 is_last_spte(iter.old_spte, iter.level))
>                 tdp_iter_next(iter);

Do you see a substantial efficiency difference between adding a
tdp_iter_next_leaf and building on for_each_tdp_pte_using_root with
something like:

#define for_each_tdp_leaf_pte_using_root(_iter, _root, _start, _end)    \
        for_each_tdp_pte_using_root(_iter, _root, _start, _end)         \
                if (!is_shadow_present_pte(_iter.old_spte) ||           \
                    !is_last_spte(_iter.old_spte, _iter.level))         \
                        continue;                                       \
                else

I agree that putting those checks in a wrapper makes the code more concise.

>
> Paolo
>