Subject: [PATCH 3/3] arm/arm64: KVM: Fix disabled distributor operation

From: [email-masked] (Christoffer Dall)
Date: Tue, 20 Oct 2015 11:44:34 +0200

On Tue, Oct 20, 2015 at 11:08:44AM +0200, Eric Auger wrote:
> Hi Christoffer,
> On 10/17/2015 10:30 PM, Christoffer Dall wrote:
> > We currently do a single update of the vgic state when the distrbutor
> distributor
> > enable/disable control register is accessed and then bypass updating the
> > state for as long as the distributor remains disabled.
> > 
> > This is incorrect, because updating the state does not consider the
> > distributor enable bit, and this you can end up in a situation where an
> > interrupt is marked as pending on the CPU interface, but not pending on
> > the distributor, which is an impossible state to be in, and triggers a
> > warning.  Consider for example the following sequence of events:
> > 
> > 1. An interrupt is marked as pending on the distributor
> >    - the interrupt is also forwarded to the CPU interface
> > 2. The guest turns off the distributor (it's about to do a reboot)
> >    - we stop updating the CPU interface state from now on
> > 3. The guest disables the pending interrupt
> >    - we remove the pending state from the distributor, but don't touch
> >      the CPU interface, see point 2.
> > 
> > Since the distributor disable bit really means that no interrupts should
> > be forwarded to the CPU interface, we modify the code to keep updating
> > the internal VGIC state, but always set the CPU interface pending bits
> > to zero when the distributor is disabled.
> > 
> > Signed-off-by: Christoffer Dall <[email-masked]>
> > ---
> >  virt/kvm/arm/vgic.c | 11 ++++++-----
> >  1 file changed, 6 insertions(+), 5 deletions(-)
> > 
> > diff --git a/virt/kvm/arm/vgic.c b/virt/kvm/arm/vgic.c
> > index 58b1256..66c6616 100644
> > --- a/virt/kvm/arm/vgic.c
> > +++ b/virt/kvm/arm/vgic.c
> > @@ -1012,6 +1012,12 @@ static int compute_pending_for_cpu(struct kvm_vcpu *vcpu)
> >  	pend_percpu = vcpu->arch.vgic_cpu.pending_percpu;
> >  	pend_shared = vcpu->arch.vgic_cpu.pending_shared;
> >  
> > +	if (!dist->enabled) {
> > +		bitmap_zero(pend_percpu, VGIC_NR_PRIVATE_IRQS);
> > +		bitmap_zero(pend_shared, nr_shared);
> > +		return 0;
> > +	}
> > +
> >  	pending = vgic_bitmap_get_cpu_map(&dist->irq_pending, vcpu_id);
> >  	enabled = vgic_bitmap_get_cpu_map(&dist->irq_enabled, vcpu_id);
> >  	bitmap_and(pend_percpu, pending, enabled, VGIC_NR_PRIVATE_IRQS);
> > @@ -1039,11 +1045,6 @@ void vgic_update_state(struct kvm *kvm)
> >  	struct kvm_vcpu *vcpu;
> >  	int c;
> >  
> > -	if (!dist->enabled) {
> > -		set_bit(0, dist->irq_pending_on_cpu);
> > -		return;
> I am confused. Don't you want to clear the whole bitmap?

So the first line used to set irq_pending_on_cpu for VCPU0 when the
distributor was disabled, which I think basically worked around the
guest kernel expecting to see a timer interrupt during boot when doing a
WFI.  Now when that's fixed, we don't need this (gigantuous hack) anymore.

The return statement was also weird and buggy, because it never
prevented anything from going to the CPU interface, it just stopped
updating things.

> 
> Shouldn't we also handle interrupts programmed in the LR. Spec says any
> ack should return a spurious ID. Is it what is going to happen with the
> current implementation?
> 

yes, we really should.  We should unqueue them, but I haven't seen any
bugs from this, and I feel like we're already changing a lot with short
notice, so I'd rather not distrupt anything more right now.

Besides, when we get around to redesigning this whole thing, the concept
of unqueueing goes away.

I know it sucks reviewing fixes that only fix a subset of a bad
implementation, but I'm aiming for 'slightly better than current state'
right now :)

-Christoffer

> > -	}
> > -
> >  	kvm_for_each_vcpu(c, vcpu, kvm) {
> >  		if (compute_pending_for_cpu(vcpu))
> >  			set_bit(c, dist->irq_pending_on_cpu);
> > 
>