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

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Date:   Wed, 10 Mar 2021 10:08:05 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     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@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH] KVM: x86/mmu: Skip !MMU-present SPTEs when removing SP in
 exclusive mode

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

Ben, do you plan to make this path take mmu_lock for read?  If so, this 
wouldn't be too useful IIUC.

Paolo