linux-kernel - Re: [PATCH v3 23/28] KVM: x86/mmu: Check for a REMOVED leaf SPTE before making the SPTE

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Message-ID: <CANgfPd-hZ0Epib2ZoQULhZkY1x4TFn6_wENnbGsiN9sHsHu2+Q@mail.gmail.com>
Date:   Tue, 1 Mar 2022 10:06:29 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        David Hildenbrand <david@...hat.com>,
        kvm <kvm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        David Matlack <dmatlack@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH v3 23/28] KVM: x86/mmu: Check for a REMOVED leaf SPTE
 before making the SPTE

On Fri, Feb 25, 2022 at 4:16 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Explicitly check for a REMOVED leaf SPTE prior to attempting to map
> the final SPTE when handling a TDP MMU fault.  Functionally, this is a
> nop as tdp_mmu_set_spte_atomic() will eventually detect the frozen SPTE.
> Pre-checking for a REMOVED SPTE is a minor optmization, but the real goal
> is to allow tdp_mmu_set_spte_atomic() to have an invariant that the "old"
> SPTE is never a REMOVED SPTE.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Ben Gardon <bgardon@...gle.com>

> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 4151e61245a7..1acd12bf309f 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1250,7 +1250,11 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>                 }
>         }
>
> -       if (iter.level != fault->goal_level) {
> +       /*
> +        * Force the guest to retry the access if the upper level SPTEs aren't
> +        * in place, or if the target leaf SPTE is frozen by another CPU.
> +        */
> +       if (iter.level != fault->goal_level || is_removed_spte(iter.old_spte)) {
>                 rcu_read_unlock();
>                 return RET_PF_RETRY;
>         }
> --
> 2.35.1.574.g5d30c73bfb-goog
>