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Subject: Re: [PATCH 2/2] KVM: x86/mmu: Don't step down in the TDP iterator
 when zapping all SPTEs
To:     Sean Christopherson <seanjc@google.com>
Cc:     Ben Gardon <bgardon@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <0086ef77-ce5f-3e89-0cbd-b17d4dccaacf@redhat.com>
Date:   Fri, 13 Aug 2021 18:38:54 +0200
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On 13/08/21 18:13, Sean Christopherson wrote:
> On Fri, Aug 13, 2021, Paolo Bonzini wrote:
>> On 12/08/21 19:46, Sean Christopherson wrote:
>>>>> 	if (iter->level == iter->min_level)
>>>>> 		return false;
>>>>>
>>>>> 	/*
>>>>> 	 * Reread the SPTE before stepping down to avoid traversing into page
>>>>> 	 * tables that are no longer linked from this entry.
>>>>> 	 */
>>>>> 	iter->old_spte = READ_ONCE(*rcu_dereference(iter->sptep));  \
>>>>>                                                                         ---> this is the code that is avoided
>>>>> 	child_pt = spte_to_child_pt(iter->old_spte, iter->level);   /
>>>>> 	if (!child_pt)
>>>>> 		return false;
>>>> Ah, right - so I agree with Ben that it's not too important.
>>> Ya.  There is a measurable performance improvement, but it's really only
>>> meaningful when there aren't many SPTEs to zap, otherwise the cost of zapping
>>> completely dominates the time.
>>
>> I don't understand.  When try_step_down is called by tdp_iter_next, all it
>> does is really just the READ_ONCE, because spte_to_child_pt will see a
>> non-present PTE and return immediately.  Why do two, presumably cache hot,
>> reads cause a measurable performance improvement?
> 
> It's entirely possible my measurements were bad and/or noisy.  Ah, and my kernel
> was running with CONFIG_PROVE_RCU=y, which makes the rcu_dereference() quite a bit
> more expensive.

It's one line of code and it makes sense, so I can certainly include the 
patch.  I was just a bit confused.

Paolo