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Subject: Re: [PATCH 08/10] KVM: arm/arm64: vgic: Handle unshared mapped
 interrupts
To: Christoffer Dall <cdall@linaro.org>,
        Marc Zyngier <marc.zyngier@arm.com>
References: <1495656803-28011-1-git-send-email-eric.auger@redhat.com>
 <1495656803-28011-9-git-send-email-eric.auger@redhat.com>
 <20170602133301.GD397@cbox> <23b9aee3-abf4-928e-e731-e38e729abbeb@arm.com>
 <20170602162944.GE397@cbox> <874lvqkhvv.fsf@arm.com>
 <20170608083432.GA7657@cbox>
Cc: eric.auger.pro@gmail.com, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
        alex.williamson@redhat.com, pbonzini@redhat.com,
        christoffer.dall@linaro.org, drjones@redhat.com, wei@redhat.com
From: Auger Eric <eric.auger@redhat.com>
Message-ID: <0e1d4754-a24c-2207-d969-d05c253618e9@redhat.com>
Date: Thu, 8 Jun 2017 10:55:29 +0200
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Hi Christoffer, Marc,

On 08/06/2017 10:34, Christoffer Dall wrote:
> 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.
if the line level is modeled using the physical distributor pending
state, don't you fix that case?

Thanks

Eric
> 
> Thanks,
> -Christoffer
>