Re: [PATCH 08/10] KVM: arm/arm64: vgic: Handle unshared mapped interrupts

Christoffer Dall
2017-06-02T16:26:02Z

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@redhat.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.

Perhaps we need to try to implement both and see how it looks like?

Thanks,
-Christoffer