linux-kernel - Re: [PATCH] KVM: ensure APICv is considered inactive if there is no APIC

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Date:   Tue, 30 Nov 2021 23:08:46 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Ignat Korchagin <ignat@...udflare.com>
Subject: Re: [PATCH] KVM: ensure APICv is considered inactive if there is no
 APIC

On Tue, 2021-11-30 at 20:39 +0000, Sean Christopherson wrote:
> On Tue, Nov 30, 2021, Paolo Bonzini wrote:
> > kvm_vcpu_apicv_active() returns false if a virtual machine has no in-kernel
> > local APIC, however kvm_apicv_activated might still be true if there are
> > no reasons to disable APICv; in fact it is quite likely that there is none
> > because APICv is inhibited by specific configurations of the local APIC
> > and those configurations cannot be programmed.  This triggers a WARN:
> > 
> >    WARN_ON_ONCE(kvm_apicv_activated(vcpu->kvm) != kvm_vcpu_apicv_active(vcpu));
> > 
> > To avoid this, introduce another cause for APICv inhibition, namely the
> > absence of an in-kernel local APIC.  This cause is enabled by default,
> > and is dropped by either KVM_CREATE_IRQCHIP or the enabling of
> > KVM_CAP_IRQCHIP_SPLIT.
> > 
> > Reported-by: Ignat Korchagin <ignat@...udflare.com>
> > Fixes: ee49a8932971 ("KVM: x86: Move SVM's APICv sanity check to common x86", 2021-10-22)
> > Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> > ---
> 
> Reviewed-by: Sean Christopherson <seanjc@...gle.com>
> 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 0ee1a039b490..e0aa4dd53c7f 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -5740,6 +5740,7 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> >  		smp_wmb();
> >  		kvm->arch.irqchip_mode = KVM_IRQCHIP_SPLIT;
> >  		kvm->arch.nr_reserved_ioapic_pins = cap->args[0];
> > +		kvm_request_apicv_update(kvm, true, APICV_INHIBIT_REASON_ABSENT);
> >  		r = 0;
> >  split_irqchip_unlock:
> >  		mutex_unlock(&kvm->lock);
> > @@ -6120,6 +6121,7 @@ long kvm_arch_vm_ioctl(struct file *filp,
> >  		/* Write kvm->irq_routing before enabling irqchip_in_kernel. */
> >  		smp_wmb();
> >  		kvm->arch.irqchip_mode = KVM_IRQCHIP_KERNEL;
> > +		kvm_request_apicv_update(kvm, true, APICV_INHIBIT_REASON_ABSENT);
> 
> Blech, kvm_request_apicv_update() is very counter-intuitive, true == clear. :-/
> Wrappers along the lines of kvm_{set,clear}_apicv_inhibit() would help a lot, and
> would likely avoid a handful of newlines as well.  I'll send a patch on top of this,
> unless you want to do it while pushing this one out.

100% agree that kvm_request_apicv_update() is very counter-intuitive!

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky

> 
> >  	create_irqchip_unlock:
> >  		mutex_unlock(&kvm->lock);
> >  		break;
> > @@ -8818,10 +8820,9 @@ static void kvm_apicv_init(struct kvm *kvm)
> >  {
> >  	init_rwsem(&kvm->arch.apicv_update_lock);
> >  
> > -	if (enable_apicv)
> > -		clear_bit(APICV_INHIBIT_REASON_DISABLE,
> > -			  &kvm->arch.apicv_inhibit_reasons);
> > -	else
> > +	set_bit(APICV_INHIBIT_REASON_ABSENT,
> > +		&kvm->arch.apicv_inhibit_reasons);
> 
> Nit, this one fits on a single line.
> 
> > +	if (!enable_apicv)
> >  		set_bit(APICV_INHIBIT_REASON_DISABLE,
> >  			&kvm->arch.apicv_inhibit_reasons);
> >  }
> > -- 
> > 2.31.1
> >