Sender: Paolo Bonzini <paolo.bonzini@gmail.com>
Message-ID: <e1c4ec6a-7c1e-b96c-63e6-d07b35820def@redhat.com>
Date:   Wed, 8 Dec 2021 15:43:19 +0100
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Subject: Re: [PATCH v3 21/26] KVM: SVM: Drop AVIC's intermediate
 avic_set_running() helper
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To:     Sean Christopherson <seanjc@google.com>,
        Joerg Roedel <joro@8bytes.org>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
        kvm@vger.kernel.org, iommu@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, Maxim Levitsky <mlevitsk@redhat.com>
References: <20211208015236.1616697-1-seanjc@google.com>
 <20211208015236.1616697-22-seanjc@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <20211208015236.1616697-22-seanjc@google.com>
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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);
> +