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

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Message-ID: <5dc72eec-a4bd-f31a-f439-cdf8c5b48c05@redhat.com>
Date:   Wed, 7 Oct 2020 19:21:30 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.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 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.

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.

Paolo