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Date:   Mon, 25 Oct 2021 16:34:43 +0200
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Subject: Re: [PATCH v2 39/43] KVM: VMX: Don't do full kick when triggering
 posted interrupt "fails"
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To:     Sean Christopherson <seanjc@google.com>,
        Marc Zyngier <maz@kernel.org>,
        Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Anup Patel <anup.patel@wdc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>
Cc:     James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Atish Patra <atish.patra@wdc.com>,
        David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-mips@vger.kernel.org, kvm@vger.kernel.org,
        kvm-ppc@vger.kernel.org, kvm-riscv@lists.infradead.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        David Matlack <dmatlack@google.com>,
        Oliver Upton <oupton@google.com>,
        Jing Zhang <jingzhangos@google.com>
References: <20211009021236.4122790-1-seanjc@google.com>
 <20211009021236.4122790-40-seanjc@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <20211009021236.4122790-40-seanjc@google.com>
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On 09/10/21 04:12, Sean Christopherson wrote:
> +		/*
> +		 * The smp_wmb() in kvm_make_request() pairs with the smp_mb_*()
> +		 * after setting vcpu->mode in vcpu_enter_guest(), thus the vCPU
> +		 * is guaranteed to see the event request if triggering a posted
> +		 * interrupt "fails" because vcpu->mode != IN_GUEST_MODE.

This explanation doesn't make much sense to me.  This is just the usual 
request/kick pattern explained in 
Documentation/virt/kvm/vcpu-requests.rst; except that we don't bother 
with a "kick" out of guest mode because the entry always goes through 
kvm_check_request (in the nVMX case) or sync_pir_to_irr (if non-nested) 
and completes the delivery itself.

In other word, it is a similar idea as patch 43/43.

What this smp_wmb() pair with, is the smp_mb__after_atomic in 
kvm_check_request(KVM_REQ_EVENT, vcpu).  Setting the interrupt in the 
PIR orders before kvm_make_request in this thread, and orders after 
kvm_make_request in the vCPU thread.

Here, instead:

> +	/*
> +	 * The implied barrier in pi_test_and_set_on() pairs with the smp_mb_*()
> +	 * after setting vcpu->mode in vcpu_enter_guest(), thus the vCPU is
> +	 * guaranteed to see PID.ON=1 and sync the PIR to IRR if triggering a
> +	 * posted interrupt "fails" because vcpu->mode != IN_GUEST_MODE.
> +	 */
>  	if (vcpu != kvm_get_running_vcpu() &&
>  	    !kvm_vcpu_trigger_posted_interrupt(vcpu, false))
> -		kvm_vcpu_kick(vcpu);
> +		kvm_vcpu_wake_up(vcpu);
>  

it pairs with the smp_mb__after_atomic in vmx_sync_pir_to_irr().  As 
explained again in vcpu-requests.rst, the ON bit has the same function 
as vcpu->request in the previous case.

Paolo

> +		 */
>   		kvm_make_request(KVM_REQ_EVENT, vcpu);