linux-kernel - Re: [PATCH v2 6/6] KVM: nVMX: optimize prepare_vmcs02{,

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Date:   Wed, 25 Jul 2018 14:55:10 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>, linux-kernel@...r.kernel.org,
        Jim Mattson <jmattson@...gle.com>,
        Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH v2 6/6] KVM: nVMX: optimize prepare_vmcs02{,_full} for
 Enlightened VMCS case

On 25/07/2018 14:50, Vitaly Kuznetsov wrote:
>>
>> But is L0 allowed to write to hv_clean_fields?  
> It is kinda expected to: currently I reset it in vmx_vcpu_run() and (if
> I remember correctly) L1 Hyper-V only clears bits in this mask when it
> touches certain fields so if we don't set it to 'all clean' it stays
> zeroed forever.

Oh, good.  I didn't understand it was bidirectional.

 So nothing stops us from doing
> 
>        if (hv_evmcs && vmx->nested.dirty_vmcs12)
>                hv_evmcs->hv_clean_fields &=
>                        ~HV_VMX_ENLIGHTENED_CLEAN_FIELD_ALL;
> 
> in prepare_vmcs02() I guess.

In prepare_vmcs02, or rather in the enlightened VMPTRLD?

>> One possibility is to
>> add a dirty_evmcs field to struct nested_vmx, and "OR" ~hv_clean_fields
>> into it at the beginning of prepare_vmcs02.
>>
>> Something like
>>
>> 	if (vmx->nested.hv_evmcs) {
>> 		vmx->nested.dirty_evmcs |=
>> 			~vmx->nested.hv_evmcs->hv_clean_fields;
>> 		prepare_vmcs02_full(vcpu, vmcs12,
>> 				    vmx->nested.dirty_evmcs);
>> 	} else if (vmx->nested.dirty_vmcs12) {
>> 		prepare_vmcs02_full(vcpu, vmcs12, ~0);
>> 	}
>>
>> 	...
>> 	vmx->nested.dirty_evmcs = 0;
>> 	vmx->nested.dirty_vmcs12 = false;
>>
>> ?
>>
> I think we can even get away with a local variable in prepare_vmcs02()
> and pass it to prepare_vmcs02_full(), no need to have it in struct
> nested_vmx. But I would slightly prefer to just reset
> hv_evmcs->hv_clean_fields when vmcs12 is dirty.

Yes, that's even better.

Paolo