linux-kernel - Re: AVIC related warning in enable_irq

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Date:   Mon, 4 May 2020 17:37:44 +0700
From:   Suravee Suthikulpanit <suravee.suthikulpanit@....com>
To:     Paolo Bonzini <pbonzini@...hat.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

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.

  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