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

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Date:   Wed, 25 Jul 2018 15:26:12 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...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

Paolo Bonzini <pbonzini@...hat.com> writes:

> 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?
>

Doing it in nested_vmx_handle_enlightened_vmptrld() is even better: we
can simplify copy_enlightened_to_vmcs12() too!

The other place where we set dirty_vmcs12 is the newly introduced
vmx_set_nested_state() but I think I'm going to add support for eVMCS
there later and just return something like -ENOTSUPP for now. Too many
people work on nested simultaneously :-)

-- 
  Vitaly