linux-kernel - Re: [PATCH 10/13] KVM: x86/mmu: Allow zapping collapsible SPTEs to use MMU read lock

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Date:   Thu, 1 Apr 2021 12:12:52 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 10/13] KVM: x86/mmu: Allow zapping collapsible SPTEs to
 use MMU read lock

On 31/03/21 23:08, Ben Gardon wrote:
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index dcbfc784cf2f..81967b4e7d76 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5610,10 +5610,13 @@ void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
>   
>   	write_lock(&kvm->mmu_lock);
>   	slot_handle_leaf(kvm, slot, kvm_mmu_zap_collapsible_spte, true);
> -
> -	if (is_tdp_mmu_enabled(kvm))
> -		kvm_tdp_mmu_zap_collapsible_sptes(kvm, slot);
>   	write_unlock(&kvm->mmu_lock);
> +
> +	if (is_tdp_mmu_enabled(kvm)) {
> +		read_lock(&kvm->mmu_lock);
> +		kvm_tdp_mmu_zap_collapsible_sptes(kvm, memslot);
> +		read_unlock(&kvm->mmu_lock);
> +	}
>   }

Same here, this will conflict with

KVM: x86/mmu: Coalesce TLB flushes when zapping collapsible SPTEs

Again, you will have to add back some "if (flush)" before the write_unlock.

Paolo