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: <218d4dbd-20f1-5bf8-ca44-c53dd9345dab@redhat.com>
Date:   Wed, 18 Mar 2020 11:36:04 +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 19:22, Sean Christopherson wrote:
> On Tue, Mar 17, 2020 at 06:18:37PM +0100, Paolo Bonzini wrote:
>> 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?
> 
> Now that you point it out, I think it makes sense to unconditionally pass
> %true here, i.e. rely 100% on nested_vmx_transition_tlb_flush() to do the
> right thing.

Why doesn't it need KVM_REQ_MMU_SYNC either?  All this should be in a
comment as well, of course.

(All patches I didn't comment on look good).

Paolo