linux-kernel - Re: [PATCH v3 34/37] KVM: nVMX: Don't flush TLB on nested VMX transition

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Date:   Tue, 24 Mar 2020 12:20:31 +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 v3 34/37] KVM: nVMX: Don't flush TLB on nested VMX
 transition

On 20/03/20 22:28, Sean Christopherson wrote:
> Unconditionally skip the TLB flush triggered when reusing a root for a
> nested transition as nested_vmx_transition_tlb_flush() ensures the TLB
> is flushed when needed, regardless of whether the MMU can reuse a cached
> root (or the last root).
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>

So much for my WARN_ON. :)

Paolo

> ---
>  arch/x86/kvm/mmu/mmu.c    | 2 +-
>  arch/x86/kvm/vmx/nested.c | 6 ++++--
>  2 files changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 84e1e748c2b3..7b0fb7f2c24d 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5038,7 +5038,7 @@ void kvm_init_shadow_ept_mmu(struct kvm_vcpu *vcpu, bool execonly,
>  		kvm_calc_shadow_ept_root_page_role(vcpu, accessed_dirty,
>  						   execonly, level);
>  
> -	__kvm_mmu_new_cr3(vcpu, new_eptp, new_role.base, false, true);
> +	__kvm_mmu_new_cr3(vcpu, new_eptp, new_role.base, true, true);
>  
>  	if (new_role.as_u64 == context->mmu_role.as_u64)
>  		return;
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index db3ce8f297c2..92aab4166498 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -1161,10 +1161,12 @@ static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3, bool ne
>  	}
>  
>  	/*
> -	 * See nested_vmx_transition_mmu_sync for details on skipping the MMU sync.
> +	 * Unconditionally skip the TLB flush on fast CR3 switch, all TLB
> +	 * flushes are handled by nested_vmx_transition_tlb_flush().  See
> +	 * nested_vmx_transition_mmu_sync for details on skipping the MMU sync.
>  	 */
>  	if (!nested_ept)
> -		kvm_mmu_new_cr3(vcpu, cr3, false,
> +		kvm_mmu_new_cr3(vcpu, cr3, true,
>  				!nested_vmx_transition_mmu_sync(vcpu));
>  
>  	vcpu->arch.cr3 = cr3;
>