linux-kernel - Re: [PATCH] KVM: x86/mmu: Skip !MMU-present SPTEs when removing SP in exclusive mode

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Date:   Fri, 12 Mar 2021 19:12:14 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/mmu: Skip !MMU-present SPTEs when removing SP in
 exclusive mode

On 10/03/21 23:24, Ben Gardon wrote:
> On Wed, Mar 10, 2021 at 1:14 PM Sean Christopherson <seanjc@...gle.com> wrote:
>>
>> On Wed, Mar 10, 2021, Paolo Bonzini wrote:
>>> On 10/03/21 01:30, Sean Christopherson wrote:
>>>> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
>>>> index 50ef757c5586..f0c99fa04ef2 100644
>>>> --- a/arch/x86/kvm/mmu/tdp_mmu.c
>>>> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
>>>> @@ -323,7 +323,18 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, u64 *pt,
>>>>                              cpu_relax();
>>>>                      }
>>>>              } else {
>>>> +                   /*
>>>> +                    * If the SPTE is not MMU-present, there is no backing
>>>> +                    * page associated with the SPTE and so no side effects
>>>> +                    * that need to be recorded, and exclusive ownership of
>>>> +                    * mmu_lock ensures the SPTE can't be made present.
>>>> +                    * Note, zapping MMIO SPTEs is also unnecessary as they
>>>> +                    * are guarded by the memslots generation, not by being
>>>> +                    * unreachable.
>>>> +                    */
>>>>                      old_child_spte = READ_ONCE(*sptep);
>>>> +                   if (!is_shadow_present_pte(old_child_spte))
>>>> +                           continue;
>>>>                      /*
>>>>                       * Marking the SPTE as a removed SPTE is not
> 
> This optimization also makes me think we could also skip the
> __handle_changed_spte call in the read mode case if the SPTE change
> was !PRESENT -> REMOVED.
> 
Yes, I think so.  It should be a separate patch anyway, so I've queued 
this one.

Paolo