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

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Date:   Wed, 7 Oct 2020 10:28:29 -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 Wed, Oct 7, 2020 at 10:21 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 07/10/20 18:30, Ben Gardon wrote:
> >> 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.
> >
>
> No, that would be just another way to write the same thing.  That said,
> making the iteration API more complicated also has disadvantages because
> if get a Cartesian explosion of changes.

I wouldn't be too worried about that. The only things I ever found
worth making an iterator case for were:
Every SPTE
Every present SPTE
Every present leaf SPTE

And really there aren't many cases that use the middle one.

>
> Regarding the naming, I'm leaning towards
>
>     tdp_root_for_each_pte
>     tdp_vcpu_for_each_pte
>
> which is shorter than the version with "using" and still clarifies that
> "root" and "vcpu" are the thing that the iteration works on.

That sounds good to me. I agree it's similarly clear.

>
> Paolo
>