Re: [PATCH 05/10] KVM: arm/arm64: vgic: Relax vgic_can_sample_irq for edge IRQs
On 3 July 2015 at 10:50, Marc Zyngier marc.zyng...@arm.com wrote: On 02/07/15 17:23, Christoffer Dall wrote: If we had a different *shared* device than the timer which is edge-triggered, don't we then also need to capture the physical distributor's pending state along with the state of the device unless we assume that upon restoring the state for the device count on the device to have another rising/falling edge to trigger the interrupt again? (I assume the line would always go high for a level-triggered interrupt in this case). I'd definitely assume that restoring the state of the device would make it generate an interrupt. This has to be a property of the device, otherwise it is not really shareable between vcpus. FWIW, QEMU's modelling approach to this is to say that devices do *not* generate interrupts on restore. If the device had previously generated an interrupt then this should be captured by the state of the interrupt controller (or whatever else it is connected to) and dealt with when the GIC state is restored. If you say that restoring the device state is supposed to generate an interrupt, you introduce an ordering requirement that the state of the interrupt controller is restored first and the device second (otherwise the incoming GIC state will overwrite the interrupt that the device just generated), which isn't ideal (especially since QEMU makes no guarantees about restore order between devices). thanks -- PMM ___ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
Re: [PATCH 05/10] KVM: arm/arm64: vgic: Relax vgic_can_sample_irq for edge IRQs
On Wed, Jul 01, 2015 at 07:18:40PM +0100, Marc Zyngier wrote: On 01/07/15 12:58, Christoffer Dall wrote: On Wed, Jul 01, 2015 at 10:17:52AM +0100, Marc Zyngier wrote: On 30/06/15 21:19, Christoffer Dall wrote: On Mon, Jun 08, 2015 at 06:04:00PM +0100, Marc Zyngier wrote: We only set the irq_queued flag for level interrupts, meaning that !vgic_irq_is_queued(vcpu, irq) is a good enough predicate for all interrupts. This will allow us to inject edge HW interrupts, for which the state ACTIVE+PENDING is not allowed. I don't understand this; ACTIVE+PENDING is allowed for edge interrupts. Do you mean that if we set the HW bit in the LR, then we are linking to an HW interrupt where we don't allow that to be ACTIVE+PENDING on the HW GIC side? Why is this relevant here? I feel like I'm missing context. I've probably taken a shortcut here - bear with me while I'm trying to explain the issue. For HW interrupts, we shouldn't even try to use the state bits in the LR, because that state is contained in the physical distributor. Setting the HW bit really means there is something going on at the distributor level, just go there. ok, so by HW interrupts you mean virtual interrupts with the HW bit in the LR set, correct? Yes, sorry. If we were to inject a ACTIVE+PENDING interrupt at the LR level, we'd basically loose the second interrupt because that state is simply not considered. Huh? Which second interrupt. I looked at the spec and it says don't use the state bits for HW interrupts, so isn't it simply not supported to set these bits at all and that's it? I managed to confuse myself reading the same bit. It says (GICv3 spec): A hypervisor must only use the pending and active state for software originated interrupts, which are typically associated with virtual devices, or SGIs. That's the PENDING+ACTIVE state, and not the pending and active bits like I read it initially. Now consider the following scenario: - We inject a virtual edge interrupt - We mark the corresponding physical interrupt as active. - Queue interrupt in an LR - Resume vcpu Now, we inject another edge interrupt, the vcpu exits for whatever reason, and the previously injected interrupt is still active. The normal vGIC flow would be to mark the interrupt as ACTIVE+PENDING in the LR, and resume the vcpu. But the above states that this is invalid for HW generated interrupts. Right, ok, so we must resample the pending state even for an edge-triggered interrupt once it's EOIed, because we cannot put it in the LR despite it being pending on the physical distributor? Incidentally, we do not need to set the EOI_INT bit, becuase when the guest EOIs the interrupt, it will also deactivate it on the physical distributor and the hardware will then take the pending physical interrupt, we will handle it in the host, etc. etc. If we had a different *shared* device than the timer which is edge-triggered, don't we then also need to capture the physical distributor's pending state along with the state of the device unless we assume that upon restoring the state for the device count on the device to have another rising/falling edge to trigger the interrupt again? (I assume the line would always go high for a level-triggered interrupt in this case). So the trick we're using is to only inject the active interrupt, and prevent anything else from being injected until we can confirm that the active state has been cleared at the physical level. Does it make any sense? Sort of, but what I don't understand now is how the guest ever sees the interrupt then. If we always inject the virtual interrupt by setting the active state on the physical distributor, and we can't inject this as active+pending, and the guest doesn't see the state in the LR, then how does this ever raise a virtual interrupt and how does the guest see an interrupt which is only PENDING so that it can ack it etc. etc.? Maybe I don't fully understand how the HW bit works after all... The way the spec is written is slightly misleading. But the gist of it is that we still signal the guest using the PENDING bit in the LR, and switch the LR as usual. it is just that we can't use the PENDING+ACTIVE state (apparently, this can lead to a double deactivation). Not sure the above makes sense. Beer time, I suppose. It does make sense, I just had to sleep on it and see the code as a whole instead of trying to understand it by just looking at this patch individually. Thanks, -Christoffer ___ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
Re: [PATCH 05/10] KVM: arm/arm64: vgic: Relax vgic_can_sample_irq for edge IRQs
On Wed, Jul 01, 2015 at 10:17:52AM +0100, Marc Zyngier wrote: On 30/06/15 21:19, Christoffer Dall wrote: On Mon, Jun 08, 2015 at 06:04:00PM +0100, Marc Zyngier wrote: We only set the irq_queued flag for level interrupts, meaning that !vgic_irq_is_queued(vcpu, irq) is a good enough predicate for all interrupts. This will allow us to inject edge HW interrupts, for which the state ACTIVE+PENDING is not allowed. I don't understand this; ACTIVE+PENDING is allowed for edge interrupts. Do you mean that if we set the HW bit in the LR, then we are linking to an HW interrupt where we don't allow that to be ACTIVE+PENDING on the HW GIC side? Why is this relevant here? I feel like I'm missing context. I've probably taken a shortcut here - bear with me while I'm trying to explain the issue. For HW interrupts, we shouldn't even try to use the state bits in the LR, because that state is contained in the physical distributor. Setting the HW bit really means there is something going on at the distributor level, just go there. ok, so by HW interrupts you mean virtual interrupts with the HW bit in the LR set, correct? If we were to inject a ACTIVE+PENDING interrupt at the LR level, we'd basically loose the second interrupt because that state is simply not considered. Huh? Which second interrupt. I looked at the spec and it says don't use the state bits for HW interrupts, so isn't it simply not supported to set these bits at all and that's it? So the trick we're using is to only inject the active interrupt, and prevent anything else from being injected until we can confirm that the active state has been cleared at the physical level. Does it make any sense? Sort of, but what I don't understand now is how the guest ever sees the interrupt then. If we always inject the virtual interrupt by setting the active state on the physical distributor, and we can't inject this as active+pending, and the guest doesn't see the state in the LR, then how does this ever raise a virtual interrupt and how does the guest see an interrupt which is only PENDING so that it can ack it etc. etc.? Maybe I don't fully understand how the HW bit works after all... Thanks, -Christoffer ___ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
Re: [PATCH 05/10] KVM: arm/arm64: vgic: Relax vgic_can_sample_irq for edge IRQs
On 30/06/15 21:19, Christoffer Dall wrote: On Mon, Jun 08, 2015 at 06:04:00PM +0100, Marc Zyngier wrote: We only set the irq_queued flag for level interrupts, meaning that !vgic_irq_is_queued(vcpu, irq) is a good enough predicate for all interrupts. This will allow us to inject edge HW interrupts, for which the state ACTIVE+PENDING is not allowed. I don't understand this; ACTIVE+PENDING is allowed for edge interrupts. Do you mean that if we set the HW bit in the LR, then we are linking to an HW interrupt where we don't allow that to be ACTIVE+PENDING on the HW GIC side? Why is this relevant here? I feel like I'm missing context. I've probably taken a shortcut here - bear with me while I'm trying to explain the issue. For HW interrupts, we shouldn't even try to use the state bits in the LR, because that state is contained in the physical distributor. Setting the HW bit really means there is something going on at the distributor level, just go there. If we were to inject a ACTIVE+PENDING interrupt at the LR level, we'd basically loose the second interrupt because that state is simply not considered. So the trick we're using is to only inject the active interrupt, and prevent anything else from being injected until we can confirm that the active state has been cleared at the physical level. Does it make any sense? M. -- Jazz is not dead. It just smells funny... ___ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
[PATCH 05/10] KVM: arm/arm64: vgic: Relax vgic_can_sample_irq for edge IRQs
We only set the irq_queued flag for level interrupts, meaning
that !vgic_irq_is_queued(vcpu, irq) is a good enough predicate
for all interrupts.
This will allow us to inject edge HW interrupts, for which the
state ACTIVE+PENDING is not allowed.
Signed-off-by: Marc Zyngier marc.zyng...@arm.com
---
virt/kvm/arm/vgic.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/virt/kvm/arm/vgic.c b/virt/kvm/arm/vgic.c
index 78fb820..59ed7a3 100644
--- a/virt/kvm/arm/vgic.c
+++ b/virt/kvm/arm/vgic.c
@@ -377,7 +377,7 @@ void vgic_cpu_irq_clear(struct kvm_vcpu *vcpu, int irq)
static bool vgic_can_sample_irq(struct kvm_vcpu *vcpu, int irq)
{
- return vgic_irq_is_edge(vcpu, irq) || !vgic_irq_is_queued(vcpu, irq);
+ return !vgic_irq_is_queued(vcpu, irq);
}
/**
--
2.1.4
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