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SubjectRe: [PATCH 2/2] KVM: x86/mmu: Don't step down in the TDP iterator when zapping all SPTEs
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On 12/08/21 19:33, Sean Christopherson wrote:
> On Thu, Aug 12, 2021, Paolo Bonzini wrote:
>> On 12/08/21 19:07, Sean Christopherson wrote:
>>> Yeah, I was/am on the fence too, I almost included a blurb in the cover letter
>>> saying as much. I'll do that for v2 and let Paolo decide.
>>
>> I think it makes sense to have it. You can even use the ternary operator
>
> Hah, yeah, I almost used a ternary op. Honestly don't know why I didn't, guess
> my brain flipped a coin.
>
>>
>> /*
>> * When zapping everything, all entries at the top level
>> * ultimately go away, and the levels below go down with them.
>> * So do not bother iterating all the way down to the leaves.
>
> The subtle part is that try_step_down() won't actually iterate down because it
> explicitly rereads and rechecks the SPTE.
>
> 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.

Paolo

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