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

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Message-ID: <YXrYo9mtueDT0bnu@google.com>
Date:   Thu, 28 Oct 2021 17:06:43 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...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>,
        Paolo Bonzini <pbonzini@...hat.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 Thu, Oct 28, 2021, Maxim Levitsky wrote:
> On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote:
> > Signal the AVIC doorbell iff the vCPU is running in the guest.  If the vCPU
> > is not IN_GUEST_MODE, it's guaranteed to pick up any pending IRQs on the
> > next VMRUN, which unconditionally processes the vIRR.
> > 
> > Add comments to document the logic.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  arch/x86/kvm/svm/avic.c | 14 ++++++++++++--
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> > index 208c5c71e827..cbf02e7e20d0 100644
> > --- a/arch/x86/kvm/svm/avic.c
> > +++ b/arch/x86/kvm/svm/avic.c
> > @@ -674,7 +674,12 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
> >  	kvm_lapic_set_irr(vec, vcpu->arch.apic);
> >  	smp_mb__after_atomic();
> >  
> > -	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);
> >  
> >  		/*
> > @@ -687,8 +692,13 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
> >  		if (cpu != get_cpu())
> >  			wrmsrl(SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpu));
> >  		put_cpu();
> > -	} else
> > +	} else {
> > +		/*
> > +		 * Wake the vCPU if it was blocking.  KVM will then detect the
> > +		 * pending IRQ when checking if the vCPU has a wake event.
> > +		 */
> >  		kvm_vcpu_wake_up(vcpu);
> > +	}
> >  
> >  	return 0;
> >  }
> 
> It makes sense indeed to avoid ringing the doorbell when the vCPU is not in
> the guest mode.
> 
> I do wonder if we want to call kvm_vcpu_wake_up always otherwise, as the vCPU
> might be just outside of the guest mode and not scheduled out. I don't know
> how expensive is kvm_vcpu_wake_up in this case.

IIUC, you're asking if we should do something like:

	if (vcpu->mode == IN_GUEST_MODE) {
		<signal doorbell>
	} else if (!is_vcpu_loaded(vcpu)) {
		kvm_vcpu_wake_up();
	}

The answer is that kvm_vcpu_wake_up(), which is effectively rcuwait_wake_up(),
is very cheap except for specific configurations that may or may not be valid for
production[*].  Practically speaking, is_vcpu_loaded() doesn't exist and should
never exist because it's inherently racy.  The closest we have would be

	else if (vcpu != kvm_get_running_vcpu()) {
		kvm_vcpu_wake_up();
	}

but that's extremely unlikely to be a net win because getting the current vCPU
requires atomics to disable/re-enable preemption, especially if rcuwait_wake_up()
is modified to avoid the rcu lock/unlock.

TL;DR: rcuwait_wake_up() is cheap, and if it's too expensive, a better optimization
would be to make it less expensive.

[*] https://lkml.kernel.org/r/20211020110638.797389-1-pbonzini@redhat.com
 
> Before this patch, the avic_vcpu_is_running would only be false when the vCPU
> is scheduled out (e.g when vcpu_put was done on it)
> 
> Best regards,
> 	Maxim Levitsky
>