linux-kernel - Re: [PATCH 5/7] KVM: nVMX: Reset eVMCS clean fields data from prepare

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Date:   Wed, 12 May 2021 09:19:40 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org, Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 5/7] KVM: nVMX: Reset eVMCS clean fields data from
 prepare_vmcs02()

Sean Christopherson <seanjc@...gle.com> writes:

> On Tue, May 11, 2021, Vitaly Kuznetsov wrote:
>> When nested state migration happens during L1's execution, it
>> is incorrect to modify eVMCS as it is L1 who 'owns' it at the moment.
>> At lease genuine Hyper-v seems to not be very happy when 'clean fields'
>> data changes underneath it.
>> 
>> 'Clean fields' data is used in KVM twice: by copy_enlightened_to_vmcs12()
>> and prepare_vmcs02_rare() so we can reset it from prepare_vmcs02() instead.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  arch/x86/kvm/vmx/nested.c | 9 ++++++---
>>  1 file changed, 6 insertions(+), 3 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>> index 3257a2291693..1661e2e19560 100644
>> --- a/arch/x86/kvm/vmx/nested.c
>> +++ b/arch/x86/kvm/vmx/nested.c
>> @@ -2090,9 +2090,6 @@ void nested_sync_vmcs12_to_shadow(struct kvm_vcpu *vcpu)
>>  
>>  	if (vmx->nested.hv_evmcs) {
>>  		copy_vmcs12_to_enlightened(vmx);
>> -		/* All fields are clean */
>> -		vmx->nested.hv_evmcs->hv_clean_fields |=
>> -			HV_VMX_ENLIGHTENED_CLEAN_FIELD_ALL;
>>  	} else {
>>  		copy_vmcs12_to_shadow(vmx);
>>  	}
>
> Looks like this makes both paths single lines, i.e. the curly braces can be
> dropped.
>

Yea, I've noticed that but I wasn't exactly sure the benefit of removing
one line here overwhelms the pain of needing an extra step while doing
code archeology with 'git blame' :-) Will drop on v2.

>> @@ -2636,6 +2633,12 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
>>  
>>  	kvm_rsp_write(vcpu, vmcs12->guest_rsp);
>>  	kvm_rip_write(vcpu, vmcs12->guest_rip);
>> +
>> +	/* Mark all fields as clean so L1 hypervisor can set what's dirty */
>> +	if (hv_evmcs)
>> +		vmx->nested.hv_evmcs->hv_clean_fields |=
>> +			HV_VMX_ENLIGHTENED_CLEAN_FIELD_ALL;
>> +
>>  	return 0;
>>  }
>>  
>> -- 
>> 2.30.2
>> 
>

-- 
Vitaly