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

Christoffer Dall
2017-06-08T08:34:32Z

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@linaro.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@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.
>
> 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