linux-kernel - Re: AVIC related warning in enable_irq

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Date:   Mon, 4 May 2020 11:25:31 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: AVIC related warning in enable_irq_window

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