Re: [PATCH 3/3] KVM: SVM: Add irqchip_split() checks before enabling AVIC

[Suravee Suthikulpanit]

Hi Radim,

On 9/8/17 08:53, Radim KrÄmÃÅ wrote:
> 2017-09-05 22:39-0500, Suravee Suthikulpanit:
> > SVM AVIC hardware accelerates guest write to APIC_EOI register
> > (for edge-trigger interrupt), which means it does not trap to KVM.
> >
> > So, only enable SVM AVIC only in split irqchip mode.
> > (e.g. launching qemu w/ option '-machine kernel_irqchip=split').
>
> Yeah, hacking TMR to get the VM exit could result in future bugs.
> We have to push split irqchip as the deafult in userspaces with this
> change.

Actually, I'm not quite sure about the advantages/disadvantages with split 
irqchip, and how it would affect other cases, and why it was not used as default 
currently.

> > Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
> > ---
> >  arch/x86/kvm/svm.c | 13 +++++++++----
> >  1 file changed, 9 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > index d1b3eb4..7ce191b 100644
> > --- a/arch/x86/kvm/svm.c
> > +++ b/arch/x86/kvm/svm.c
> > @@ -1292,7 +1291,7 @@ static void init_vmcb(struct vcpu_svm *svm)
> >  		set_intercept(svm, INTERCEPT_PAUSE);
> >  	}
> >
> > -	if (avic)
> > +	if (kvm_vcpu_apicv_active(&svm->vcpu))
> >  		avic_init_vmcb(svm);
> >  	/*
> > @@ -1594,6 +1593,12 @@ static int avic_init_vcpu(struct vcpu_svm *svm)
> >  	if (!avic)
> >  		return 0;
> >
> > +	if (!irqchip_split(svm->vcpu.kvm)) {
>
> The other place used kvm_vcpu_apicv_active() instead of checking
> irqchip_split() directly, so I think it would be better to be consistent
> and do it here too.
> I'd also like if this workaround used !irqchip_split() exactly once.

Ok, I'll clean up in V2.

> > +		pr_debug("%s: Disable AVIC due to non-split irqchip.\n",
> > +			 __func__);
>
> There is going to be too much of those.  pr_debug_once() would be a
> better notification.  We can also report it in svm_get_enable_apicv().

pr_debug_once does not use dynamic debug APIs. I think I can call pr_debug only 
when vcpu_id == 0.

> > +		return 0;
> > +	}
> > +
> >  	ret = avic_init_backing_page(&svm->vcpu);
> >  	if (ret)
> >  		return ret;
> > @@ -4388,7 +4393,7 @@ static void svm_set_virtual_x2apic_mode(struct kvm_vcpu *vcpu, bool set)
> >
> >  static bool svm_get_enable_apicv(struct kvm_vcpu *vcpu)
> >  {
> > -	return avic;
> > +	return avic && irqchip_split(vcpu->kvm);
> >  }
> >
> >  static void svm_hwapic_irr_update(struct kvm_vcpu *vcpu, int max_irr)
> > @@ -4405,7 +4410,7 @@ static void svm_refresh_apicv_exec_ctrl(struct kvm_vcpu *vcpu)
> >  	struct vcpu_svm *svm = to_svm(vcpu);
> >  	struct vmcb *vmcb = svm->vmcb;
> >
> > -	if (!avic)
> > +	if (!avic || !irqchip_split(svm->vcpu.kvm))
>
> Seems like we want !kvm_vcpu_apicv_active() here too.

Right.

> >  		return;
> >
> >  	vmcb->control.int_ctl &= ~AVIC_ENABLE_MASK;
>
> (separate bug: refresh should be able to enable avic as well.)
>
> thanks.

Thanks,
Suravee