linux-kernel - Re: [PATCH v2 31/32] KVM: nVMX: Don't flush TLB on nested VM transition with EPT enabled

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Message-ID: <97f91b27-65ac-9187-6b60-184e1562d228@redhat.com>
Date:   Tue, 17 Mar 2020 18:18:37 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.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>,
        Junaid Shahid <junaids@...gle.com>,
        Liran Alon <liran.alon@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        John Haxby <john.haxby@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v2 31/32] KVM: nVMX: Don't flush TLB on nested VM
 transition with EPT enabled

On 17/03/20 05:52, Sean Christopherson wrote:
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index d816f1366943..a77eab5b0e8a 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -1123,7 +1123,7 @@ static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3, bool ne
>  	}
>  
>  	if (!nested_ept)
> -		kvm_mmu_new_cr3(vcpu, cr3, false);
> +		kvm_mmu_new_cr3(vcpu, cr3, enable_ept);

Even if enable_ept == false, we could have already scheduled or flushed
the TLB soon due to one of 1) nested_vmx_transition_tlb_flush 2)
vpid_sync_context in prepare_vmcs02 3) the processor doing it for
!enable_vpid.

So for !enable_ept only KVM_REQ_MMU_SYNC is needed, not
KVM_REQ_TLB_FLUSH_CURRENT I think.  Worth adding a TODO?

Paolo

>  	vcpu->arch.cr3 = cr3;
>  	kvm_register_mark_available(vcpu, VCPU_EXREG_CR3);
> -- 2.24.1