linux-kernel - Re: [PATCH v2 39/43] KVM: VMX: Don't do full kick when triggering posted interrupt "fails"

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Message-ID: <335822ac-b98b-1eec-4911-34e4d0e99907@redhat.com>
Date:   Mon, 25 Oct 2021 16:34:43 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>
Cc:     James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Atish Patra <atish.patra@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        David Matlack <dmatlack@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH v2 39/43] KVM: VMX: Don't do full kick when triggering
 posted interrupt "fails"

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);