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

[Marc Zyngier <marc.zyngier@arm.com>]

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.

Thanks,

	M.
-- 
Jazz is not dead, it just smell funny.