linux-kernel - Re: [PATCH 17/19] KVM: SVM: Handle multiple logical targets in AVIC kick fastpath

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Date:   Wed, 31 Aug 2022 21:25:00 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Li RongQing <lirongqing@...du.com>
Subject: Re: [PATCH 17/19] KVM: SVM: Handle multiple logical targets in AVIC
 kick fastpath

On Wed, 2022-08-31 at 18:19 +0000, Sean Christopherson wrote:
> On Wed, Aug 31, 2022, Maxim Levitsky wrote:
> > On Wed, 2022-08-31 at 00:35 +0000, Sean Christopherson wrote:
> > > +static void avic_kick_vcpu_by_logical_id(struct kvm *kvm, u32 *avic_logical_id_table,
> > > +					 u32 logid_index, u32 icrl)
> > > +{
> > > +	u32 physical_id;
> > > +
> > > +	if (!avic_logical_id_table) {
> > ^ Typo, the '!' shoudn't be there.
> 
> Ouch.  I suspect the tests pass because this just ends up routing events through
> the slow path.  I try to concoct a testcase to expose this bug.
> 
> > > +static bool is_optimized_logical_map_enabled(struct kvm *kvm)
> > > +{
> > > +	struct kvm_apic_map *map;
> > > +	bool enabled;
> > > +
> > > +	rcu_read_lock();
> > > +	map = rcu_dereference(kvm->arch.apic_map);
> > > +	enabled = map && map->logical_mode != KVM_APIC_MODE_MAP_DISABLED;
> > > +	rcu_read_unlock();
> > > +	return enabled;
> > > +}
> > 
> > This function doesn't belong to avic, it should be in common KVM code.
> 
> I'll move it.  I'm not expecting any additional users, but I agree it belongs
> elsewhere.  Actually, might be a moot point (see below).
> 
> > > @@ -394,50 +449,27 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
> > >  		if (unlikely(!bitmap))
> > >  			return 0;
> > >  
> > > -		if (!is_power_of_2(bitmap))
> > > -			/* multiple logical destinations, use slow path */
> > > +		/*
> > > +		 * Use the slow path if more than one bit is set in the bitmap
> > > +		 * and KVM's optimized logical map is disabled to avoid kicking
> > > +		 * a vCPU multiple times.  If the optimized map is disabled, a
> > > +		 * vCPU _may_ have multiple bits set in its logical ID, i.e.
> > > +		 * may have multiple entries in the logical table.
> > > +		 */
> > > +		if (!is_power_of_2(bitmap) &&
> > > +		    !is_optimized_logical_map_enabled(kvm))
> > >  			return -EINVAL;
> > 
> > I hate to say it but there is another issue here, which I know about for a while
> > but haven't gotten yet to fix.
> > 
> > The issue is that AVIC's logical to physical map can't cover all the corner cases
> > that you discovered - it only supports the sane subset: for each cluster, and for each bit
> > in the mask, it has a physical apic id - so things like logical ids with multiple bits,
> > having same logical id for multiple vcpus and so on can't work.
> > 
> > In this case we need to either inhibit AVIC (I support this 100%),
> 
> I like the idea of inhibiting.
> 
> >  or clear its logical ID map, so all logicical IPIs VM exit, and then they
> >  can be emulated.
> > 
> > I haven't studied it formally but the code which rebuilds the AVIC's logical ID map
> > starts at 'avic_handle_ldr_update'.
> 
> I suspected there are issues here, but the new tests passed (somewhat surprisingly)
> so I stopped trying to decipher the AVIC LDR handling.
> 
> Eww.  And the VM-Exit trap logic is broken too.  If the guest updates and disables
> its LDR, SVM returns immediately and doesn't call into common APIC code, i.e. doesn't
> recalc the optimized map.  E.g. if the guest clears its LDR, the optimized map will
> be left as is and the vCPU will receive interrupts using its old LDR.
> 
> 	case APIC_LDR:
> 		if (avic_handle_ldr_update(vcpu))
> 			return 0;
> 		break;
> 
> Rather than handling this purely in AVIC code, what if we a key off of
> the optimized map being enabled?  E.g. drop the return from avic_handle_ldr_update()
> and in the kvm_recalculate_apic_map() do:
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 3b6ef36b3963..6e188010b614 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -364,6 +364,11 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
>                 cluster[ldr] = apic;
>         }
>  out:
> +       if (!new || new->logical_mode == KVM_APIC_MODE_MAP_DISABLED)
> +               kvm_set_apicv_inhibit(kvm, APICV_INHIBIT_REASON_LOGICAL_MAP_DISABLED);
> +       else
> +               kvm_clear_apicv_inhibit(kvm, APICV_INHIBIT_REASON_LOGICAL_MAP_DISABLED);
> +

This looks very good, it will even work on APICv, because the 'check_apicv_inhibit_reasons'
will not return true for this new reason (APICv IPIv I think doesn't deal with logical destination at all);

Best regards,
	Maxim Levitsky

>         old = rcu_dereference_protected(kvm->arch.apic_map,
>                         lockdep_is_held(&kvm->arch.apic_map_lock));
>         rcu_assign_pointer(kvm->arch.apic_map, new);
>