linux-kernel - Re: [PATCH v2 35/43] KVM: SVM: Signal AVIC doorbell iff vCPU is in guest mode

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Date:   Wed, 27 Oct 2021 16:08:10 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     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>,
        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 35/43] KVM: SVM: Signal AVIC doorbell iff vCPU is in
 guest mode

On Wed, Oct 27, 2021, Paolo Bonzini wrote:
> 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?

LOL, I think we have a Three^WTwo Stooges routine going on.  This patch does
remove avic_vcpu_is_running() and replaces it with the vcpu->mode check.  Or am
I completely misunderstanding what your referring to?

-       if (avic_vcpu_is_running(vcpu)) {
+       /*
+        * Signal the doorbell to tell hardware to inject the IRQ if the vCPU
+        * is in the guest.  If the vCPU is not in the guest, hardware will
+        * automatically process AVIC interrupts at VMRUN.
+        */
+       if (vcpu->mode == IN_GUEST_MODE) {
                int cpu = READ_ONCE(vcpu->cpu);