linux-kernel - Re: AVIC related warning in enable_irq

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Message-ID: <159382e7fdf0f9b50d79e29554842289e92e1ed7.camel@redhat.com>
Date:   Mon, 04 May 2020 12:13:47 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: AVIC related warning in enable_irq_window

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()?
> 
> Thanks,
> Suravee
> 

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.

Best regards,
	Maxim Levitsky