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

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Date:   Wed, 8 Dec 2021 15:43:19 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     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 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:

	/*
	 * 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, therefore vIRR will also
	 * be checked by kvm_vcpu_check_block() before blocking.  The
	 * memory barrier implicit in set_current_state orders writing
	 * 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
	 * the cause of errata #1235).
	 */

Is there any nuance that I am missing?

Paolo

> +	 */
> +	avic_vcpu_put(vcpu);
> +