Date:   Thu, 8 Jun 2017 10:34:32 +0200
From:   Christoffer Dall <cdall@...aro.org>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     Eric Auger <eric.auger@...hat.com>, eric.auger.pro@...il.com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, alex.williamson@...hat.com,
        pbonzini@...hat.com, christoffer.dall@...aro.org,
        drjones@...hat.com, wei@...hat.com
Subject: Re: [PATCH 08/10] KVM: arm/arm64: vgic: Handle unshared mapped
 interrupts

On Thu, Jun 08, 2017 at 09:23:16AM +0100, Marc Zyngier wrote:
> On Fri, Jun 02 2017 at  6:29:44 pm BST, Christoffer Dall <cdall@...aro.org> wrote:
> > On Fri, Jun 02, 2017 at 03:10:23PM +0100, Marc Zyngier wrote:
> >> On 02/06/17 14:33, Christoffer Dall wrote:
> >> > On Wed, May 24, 2017 at 10:13:21PM +0200, Eric Auger wrote:
> >> >> Virtual interrupts directly mapped to physical interrupts require
> >> >> some special care. Their pending and active state must be observed
> >> >> at distributor level and not in the list register.
> >> > 
> >> > This is not entirely true.  There's a dependency, but there is also
> >> > separate virtual vs. physical state, see below.
> >> 
> >> I think this stems for the usual confusion about the "pending and active
> >> state" vs "pending and active states". Yes, the GIC spec is rubbish. Can
> >> I state this again?
> >> 
> >> >>
> >> >> Also a level sensitive interrupt's level is not toggled down by any
> >> >> maintenance IRQ handler as the EOI is not trapped.
> >> >>
> >> >> This patch adds an host_irq field in vgic_irq struct to easily
> >> >> get the irqchip state of the host irq. We also handle the
> >> >> physical IRQ case in vgic_validate_injection and add helpers to
> >> >> get the line level and active state.
> >> >>
> >> >> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> >> >> ---
> >> >>  include/kvm/arm_vgic.h    |  4 +++-
> >> >>  virt/kvm/arm/arch_timer.c |  3 ++-
> >> >>  virt/kvm/arm/vgic/vgic.c  | 44 ++++++++++++++++++++++++++++++++++++++------
> >> >>  virt/kvm/arm/vgic/vgic.h  |  9 ++++++++-
> >> >>  4 files changed, 51 insertions(+), 9 deletions(-)
> >> >>
> >> >> diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
> >> >> index ef71858..695ebc7 100644
> >> >> --- a/include/kvm/arm_vgic.h
> >> >> +++ b/include/kvm/arm_vgic.h
> >> >> @@ -112,6 +112,7 @@ struct vgic_irq {
> >> >>  	bool hw;			/* Tied to HW IRQ */
> >> >>  	struct kref refcount;		/* Used for LPIs */
> >> >>  	u32 hwintid;			/* HW INTID number */
> >> >> +	unsigned int host_irq;		/* linux irq corresponding to hwintid */
> >> >>  	union {
> >> >>  		u8 targets;			/* GICv2 target VCPUs mask */
> >> >>  		u32 mpidr;			/* GICv3 target VCPU */
> >> >> @@ -301,7 +302,8 @@ int kvm_vgic_inject_irq(struct kvm *kvm, int cpuid, unsigned int intid,
> >> >>  			bool level);
> >> >>  int kvm_vgic_inject_mapped_irq(struct kvm *kvm, int cpuid, unsigned int intid,
> >> >>  			       bool level);
> >> >> -int kvm_vgic_map_phys_irq(struct kvm_vcpu *vcpu, u32 virt_irq, u32 phys_irq);
> >> >> +int kvm_vgic_map_phys_irq(struct kvm_vcpu *vcpu, unsigned int host_irq,
> >> >> +			  u32 virt_irq, u32 phys_irq);
> >> >>  int kvm_vgic_unmap_phys_irq(struct kvm_vcpu *vcpu, unsigned int virt_irq);
> >> >>  bool kvm_vgic_map_is_active(struct kvm_vcpu *vcpu, unsigned int virt_irq);
> >> >>  
> >> >> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> >> >> index 5976609..45f4779 100644
> >> >> --- a/virt/kvm/arm/arch_timer.c
> >> >> +++ b/virt/kvm/arm/arch_timer.c
> >> >> @@ -651,7 +651,8 @@ int kvm_timer_enable(struct kvm_vcpu *vcpu)
> >> >>  	 * Tell the VGIC that the virtual interrupt is tied to a
> >> >>  	 * physical interrupt. We do that once per VCPU.
> >> >>  	 */
> >> >> -	ret = kvm_vgic_map_phys_irq(vcpu, vtimer->irq.irq, phys_irq);
> >> >> +	ret = kvm_vgic_map_phys_irq(vcpu, host_vtimer_irq,
> >> >> +				    vtimer->irq.irq, phys_irq);
> >> >>  	if (ret)
> >> >>  		return ret;
> >> >>  
> >> >> diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
> >> >> index 83b24d2..aa0618c 100644
> >> >> --- a/virt/kvm/arm/vgic/vgic.c
> >> >> +++ b/virt/kvm/arm/vgic/vgic.c
> >> >> @@ -137,6 +137,28 @@ void vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq)
> >> >>  	kfree(irq);
> >> >>  }
> >> >>  
> >> >> +bool irq_line_level(struct vgic_irq *irq)
> >> >> +{
> >> >> +	bool line_level = irq->line_level;
> >> >> +
> >> >> +	if (unlikely(is_unshared_mapped(irq)))
> >> >> +		WARN_ON(irq_get_irqchip_state(irq->host_irq,
> >> >> +					      IRQCHIP_STATE_PENDING,
> >> >> +					      &line_level));
> >> >> +	return line_level;
> >> >> +}
> >> > 
> >> > This really looks fishy.  When do we need this exactly?
> >> > 
> >> > I feel like we should treat this more like everything else and set the
> >> > line_level on the irq even for forwarded interrupts, and then you don't
> >> > need changes to validate injection.
> >> > 
> >> > The challenge, then, is how to re-sample the line and lower the
> >> > line_level field when necessary.  Can't we simply do this in
> >> > vgic_fold_lr_state(), and if you have a forwarded interrupt which is
> >> > level triggered and the level is high, then notify the one who injected
> >> > this and tell it to adjust its line level (lower it if it changed).
> >> > 
> >> > That would follow our existing path very closely.
> >> > 
> >> > Am I missing something?
> >> 
> >> I don't think you are. I think Eric got confused because of the above.
> >> But the flow is a bit a brainfsck :-(
> >> 
> >> - Physical interrupt fires, activated, injected in the vgic
> >> - Injecting the interrupt has a very different flow from what we
> >> currently have, and follow the same pattern as an Edge interrupt
> >> (because the Pending state is kept at the physical distributor, so we
> >> cannot preserve it in the emulation).
> >> - Normal life cycle of the interrupt
> >> - The fact that the Pending bit is kept at the distributor level ensures
> >> that if it becomes pending again in the emulation, that's because the
> >> guest has deactivated the physical interrupt by doing an EOI.
> >> 
> >
> > I think there's a choice between how we choose to support this.  We can
> > either do the edge-like injection, or we can model the line_level to the
> > best of our ability (we just have to lower the line after the guest
> > exits after deactivation if it's not still pending at the physical
> > distributor).
> >
> > One question with doing this edge-like, can you ahve this scenario:
> >  1. VM runs with active virtual interrupt linked to physical
> >     interrupt.
> >  2. VM deactivates virtual+physical interrupt
> >  3. Physical interrupt fires again on the host
> >  4. The host injects the virtual interrupt as pending to the VGIC (and
> >     IPIs the VCPU etc.)
> >  5. The device lowers the physical line (another VPCU programs the
> >     device, there's some delay, or whatever)
> >  6. The VCPU now sees a pending interrupt, which is no longer pending.
> >
> > Not sure if the line-like approach really solves this, though, or if
> > getting a spurius interrupt is something we care about.
> 
> That would be a spurious interrupt indeed, but I'm not sure that's
> something the line level sampling you suggest would avoid either. There
> is a fundamental disconnect between the injection and the physical line,
> and it can only be modelled to some level of accuracy (/me curse the
> architecture again).
> 
> > Perhaps we need to try to implement both and see how it looks like?
> 
> There is definitely room for experiment, but I feel Eric should focus on
> one of them (whichever it is). Happy to help prototyping the other one
> though.
> 
That's fair.  I'm just worried about the whole "emulate level triggered
interrupts as edge triggered" thing, but as you said, the architecture
doesn't allow us to model it more accurately.

Thanks,
-Christoffer

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