linux-kernel - Re: AVIC related warning in enable_irq

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Date:   Mon, 4 May 2020 12:49:26 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: AVIC related warning in enable_irq_window

On 04/05/20 12:37, Suravee Suthikulpanit wrote:
> Paolo / Maxim,
> 
> On 5/4/20 4:25 PM, Paolo Bonzini wrote:
>> On 04/05/20 11:13, Maxim Levitsky wrote:
>>> On Mon, 2020-05-04 at 15:46 +0700, Suravee Suthikulpanit wrote:
>>>> Paolo / Maxim,
>>>>
>>>> On 5/2/20 11:42 PM, Paolo Bonzini wrote:
>>>>> On 02/05/20 15:58, Maxim Levitsky wrote:
>>>>>> The AVIC is disabled by svm_toggle_avic_for_irq_window, which calls
>>>>>> kvm_request_apicv_update, which broadcasts the
>>>>>> KVM_REQ_APICV_UPDATE vcpu request,
>>>>>> however it doesn't broadcast it to CPU on which now we are
>>>>>> running, which seems OK,
>>>>>> because the code that handles that broadcast runs on each VCPU
>>>>>> entry, thus
>>>>>> when this CPU will enter guest mode it will notice and disable the
>>>>>> AVIC.
>>>>>>
>>>>>> However later in svm_enable_vintr, there is test
>>>>>> 'WARN_ON(kvm_vcpu_apicv_active(&svm->vcpu));'
>>>>>> which is still true on current CPU because of the above.
>>>>>
>>>>> Good point!  We can just remove the WARN_ON I think.  Can you send
>>>>> a patch?
>>>>
>>>> Instead, as an alternative to remove the WARN_ON(), would it be
>>>> better to just explicitly
>>>> calling kvm_vcpu_update_apicv(vcpu) to update the apicv_active flag
>>>> right after
>>>> kvm_request_apicv_update()?
>>>>
>>> This should work IMHO, other that the fact kvm_vcpu_update_apicv will
>>> be called again,
>>> when this vcpu is entered since the KVM_REQ_APICV_UPDATE will still
>>> be pending on it.
>>> It shoudn't be a problem, and we can even add a check to do nothing
>>> when it is called
>>> while avic is already in target enable state.
>>
>> I thought about that but I think it's a bit confusing.  If we want to
>> keep the WARN_ON, Maxim can add an equivalent one to svm_vcpu_run, which
>> is even better because the invariant is clearer.
>>
>> WARN_ON((vmcb->control.int_ctl & (AVIC_ENABLE_MASK | V_IRQ_MASK))
>>     == (AVIC_ENABLE_MASK | V_IRQ_MASK));
>>
>> Paolo
>>
> 
> Quick update. I tried your suggestion as following, and it's showing the
> warning still.
> I'll look further into this.

Ok, thanks.  By the way, there is another possible cleanup: the clearing
of V_IRQ_MASK can be removed from interrupt_window_interception since it
has already called svm_clear_vintr.

Paolo

>  arch/x86/kvm/svm/svm.c | 20 +++++++++++++-------
>  1 file changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 2f379ba..142c4b9 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1368,9 +1368,6 @@ static inline void svm_enable_vintr(struct
> vcpu_svm *svm)
>  {
>         struct vmcb_control_area *control;
> 
> -       /* The following fields are ignored when AVIC is enabled */
> -       WARN_ON(kvm_vcpu_apicv_active(&svm->vcpu));
> -
>         /*
>          * This is just a dummy VINTR to actually cause a vmexit to happen.
>          * Actual injection of virtual interrupts happens through EVENTINJ.
> @@ -3322,6 +3319,11 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>                 vcpu->arch.apic->lapic_timer.timer_advance_ns)
>                 kvm_wait_lapic_expire(vcpu);
> 
> +//SURAVEE
> +       WARN_ON((svm->vmcb->control.int_ctl &
> +                (AVIC_ENABLE_MASK | V_IRQ_MASK))
> +                == (AVIC_ENABLE_MASK | V_IRQ_MASK));
> +
> 
> Suravee
>