linux-kernel - Re: [PATCH v3 21/26] KVM: SVM: Drop AVIC's intermediate avic_set

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Message-ID: <YbDSlNsY8b2O8PtM@google.com>
Date:   Wed, 8 Dec 2021 15:43:16 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        kvm@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH v3 21/26] KVM: SVM: Drop AVIC's intermediate
 avic_set_running() helper

On Wed, Dec 08, 2021, Paolo Bonzini wrote:
> On 12/8/21 02:52, Sean Christopherson wrote:
> > +	/*
> > +	 * Unload the AVIC when the vCPU is about to block,_before_  the vCPU
> > +	 * actually blocks.  The vCPU needs to be marked IsRunning=0 before the
> > +	 * final pass over the vIRR via kvm_vcpu_check_block().  Any IRQs that
> > +	 * arrive before IsRunning=0 will not signal the doorbell, i.e. it's
> > +	 * KVM's responsibility to ensure there are no pending IRQs in the vIRR
> > +	 * after IsRunning is cleared, prior to scheduling out the vCPU.
> 
> I prefer to phrase this around paired memory barriers and the usual
> store/smp_mb/load lockless idiom:

I've no objection to that, my goal is/was purely to emphasize the need to manually
process the vIRR after clearing IsRunning.
 
> 	/*
> 	 * Unload the AVIC when the vCPU is about to block, _before_
> 	 * the vCPU actually blocks.
> 	 *
> 	 * Any IRQs that arrive before IsRunning=0 will not cause an
> 	 * incomplete IPI vmexit on the source,

It's not just IPIs, the GA log will also suffer the same fate.  That's why I
didn't mention incomplete VM-Exits.  I'm certainly not opposed to that clarification,
but I don't want readers to walk away thinking this is only a problem for IPIs.

> therefore vIRR will also

"s/vIRR will/the vIRR must" to make it abundantly clear that checking the vIRR
is effectively a hard requirement.

> 	 * be checked by kvm_vcpu_check_block() before blocking.  The
> 	 * memory barrier implicit in set_current_state orders writing

set_current_state()

> 	 * IsRunning=0 before reading the vIRR.  The processor needs a
> 	 * matching memory barrier on interrupt delivery between writing
> 	 * IRR and reading IsRunning; the lack of this barrier might be

Missing the opening paranthesis.

> 	 * the cause of errata #1235).
> 	 */