linux-kernel - Re: [PATCH v3] KVM: VMX: Enable Notify VM exit

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Date:   Fri, 25 Feb 2022 20:46:05 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Chenyi Qiang <chenyi.qiang@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     Tao Xu <tao3.xu@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] KVM: VMX: Enable Notify VM exit

On 2/25/2022 7:54 PM, Paolo Bonzini wrote:
> On 2/23/22 07:24, Chenyi Qiang wrote:
>> Nested handling
>> - Nested notify VM exits are not supported yet. Keep the same notify
>>    window control in vmcs02 as vmcs01, so that L1 can't escape the
>>    restriction of notify VM exits through launching L2 VM.
>> - When L2 VM is context invalid, synthesize a nested
>>    EXIT_REASON_TRIPLE_FAULT to L1 so that L1 won't be killed due to L2's
>>    VM_CONTEXT_INVALID happens.
>>
>> Notify VM exit is defined in latest Intel Architecture Instruction Set
>> Extensions Programming Reference, chapter 9.2.
>>
>> TODO: Allow to change the window size (to enable the feature) at runtime,
>> which can make it more flexible to do management.
> 
> I only have a couple questions, any changes in response to the question
> I can do myself.
> 
>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>> index 1dfe23963a9e..f306b642c3e1 100644
>> --- a/arch/x86/kvm/vmx/nested.c
>> +++ b/arch/x86/kvm/vmx/nested.c
>> @@ -2177,6 +2177,9 @@ static void prepare_vmcs02_constant_state(struct 
>> vcpu_vmx *vmx)
>>       if (cpu_has_vmx_encls_vmexit())
>>           vmcs_write64(ENCLS_EXITING_BITMAP, INVALID_GPA);
>> +    if (notify_window >= 0)
>> +        vmcs_write32(NOTIFY_WINDOW, notify_window);
> 
> Is a value of 0 valid?  

Yes, 0 is valid. That's why there is an internal value to ensure even 0 
won't cause false positive

> Should it be changed to the recommended value of
> 128000 in hardware_setup()?
> 
>> +    case EXIT_REASON_NOTIFY:
>> +        return nested_cpu_has2(vmcs12,
>> +            SECONDARY_EXEC_NOTIFY_VM_EXITING);
> 
> This should be "return false" since you don't expose the secondary
> control to L1 (meaning, it will never be set).

Fine with either.

>> +         * L0 will synthensize a nested TRIPLE_FAULT to kill L2 when
>> +         * notify VM exit occurred in L2 and 
>> NOTIFY_VM_CONTEXT_INVALID is
>> +         * set in exit qualification. In this case, if notify VM exit
>> +         * occurred incident to delivery of a vectored event, the IDT
>> +         * vectoring info are recorded in VMCS. Drop the pending event
>> +         * in vmcs12, otherwise L1 VMM will exit to userspace with
>> +         * internal error due to delivery event.
>>           */
>> -        vmcs12_save_pending_event(vcpu, vmcs12);
>> +        if (to_vmx(vcpu)->exit_reason.basic != EXIT_REASON_NOTIFY)
>> +            vmcs12_save_pending_event(vcpu, vmcs12);
> 
> I would prefer to call out the triple fault here:
> 
>                  /*
>                   * Transfer the event that L0 or L1 may have wanted to 
> inject into
>                   * L2 to IDT_VECTORING_INFO_FIELD.
>                   *
>                   * Skip this if the exit is due to a 
> NOTIFY_VM_CONTEXT_INVALID
>                   * exit; in that case, L0 will synthesize a nested 
> TRIPLE_FAULT
>                   * vmexit to kill L2.  No IDT vectoring info is 
> recorded for
>                   * triple faults, and __vmx_handle_exit does not expect 
> it.
>                   */
>                  if (!(to_vmx(vcpu)->exit_reason.basic == 
> EXIT_REASON_NOTIFY)
>                        && kvm_test_request(KVM_REQ_TRIPLE_FAULT, vcpu))
>                          vmcs12_save_pending_event(vcpu, vmcs12);

looks good to me.

> What do you think?
> 
> Paolo
>