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Date:   Wed, 27 Oct 2021 17:36:18 +0200
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Subject: Re: [PATCH v2 35/43] KVM: SVM: Signal AVIC doorbell iff vCPU is in
 guest mode
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To:     Sean Christopherson <seanjc@google.com>
Cc:     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>,
        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>
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 <0333be2a-76d8-657a-6c82-3bb5c9ff2e3b@redhat.com>
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From:   Paolo Bonzini <pbonzini@redhat.com>
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On 27/10/21 17:06, Sean Christopherson wrote:
>> Does this still need to check the "running" flag?  That should be a strict
>> superset of vcpu->mode == IN_GUEST_MODE.
>
> No.  Signalling the doorbell when "running" is set but the vCPU is not in the
> guest is just an expensive nop.  So even if KVM were to rework its handling of
> "running" to set the flag immediately before VMRUN and clear it immediately after,
> keying off IN_GUEST_MODE and not "running" would not be wrong, just sub-optimal.
> 
> I doubt KVM will ever make the "running" flag super precise, because keeping the
> flag set when the vCPU is loaded avoids VM-Exits on other vCPUs due to undelivered
> IPIs.

Right, so should we drop the "if (running)" check in this patch, at the 
same time as it's adding the IN_GUEST_MODE check?

Paolo