Subject: Re: [PATCH] KVM: arm64: Allow in-atomic injection of SPIs

From: Marc Zyngier <[email-masked]>
Date: Mon, 08 Jun 2020 18:19:05 +0100

Hi Eric,

On 2020-06-08 17:58, Auger Eric wrote:
> Hi Marc,
> 
> On 5/26/20 6:11 PM, Marc Zyngier wrote:
>> On a system that uses SPIs to implement MSIs (as it would be
>> the case on a GICv2 system exposing a GICv2m to its guests),
>> we deny the possibility of injecting SPIs on the in-atomic
>> fast-path.
>> 
>> This results in a very large amount of context-switches
>> (roughly equivalent to twice the interrupt rate) on the host,
>> and suboptimal performance for the guest (as measured with
>> a test workload involving a virtio interface backed by vhost-net).
>> Given that GICv2 systems are usually on the low-end of the spectrum
>> performance wise, they could do without the aggravation.
>> 
>> We solved this for GICv3+ITS by having a translation cache. But
>> SPIs do not need any extra infrastructure, and can be immediately
>> injected in the virtual distributor as the locking is already
>> heavy enough that we don't need to worry about anything.
>> 
>> This halves the number of context switches for the same workload.
>> 
>> Signed-off-by: Marc Zyngier <[email-masked]>
>> ---
>>  arch/arm64/kvm/vgic/vgic-irqfd.c | 20 ++++++++++++++++----
>>  arch/arm64/kvm/vgic/vgic-its.c   |  3 +--
>>  2 files changed, 17 insertions(+), 6 deletions(-)
>> 
>> diff --git a/arch/arm64/kvm/vgic/vgic-irqfd.c 
>> b/arch/arm64/kvm/vgic/vgic-irqfd.c
>> index d8cdfea5cc96..11a9f81115ab 100644
>> --- a/arch/arm64/kvm/vgic/vgic-irqfd.c
>> +++ b/arch/arm64/kvm/vgic/vgic-irqfd.c
> There is still a comment above saying
>  * Currently only direct MSI injection is supported.

I believe this comment to be correct. There is no path other
than MSI injection that leads us here. Case in point, we only
ever inject a rising edge through this API, never a falling one.

>> @@ -107,15 +107,27 @@ int kvm_arch_set_irq_inatomic(struct 
>> kvm_kernel_irq_routing_entry *e,
>>  			      struct kvm *kvm, int irq_source_id, int level,
>>  			      bool line_status)
>>  {
>> -	if (e->type == KVM_IRQ_ROUTING_MSI && vgic_has_its(kvm) && level) {
>> +	if (!level)
>> +		return -EWOULDBLOCK;
>> +
>> +	switch (e->type) {
>> +	case KVM_IRQ_ROUTING_MSI: {
>>  		struct kvm_msi msi;
>> 
>> +		if (!vgic_has_its(kvm))
>> +			return -EINVAL;
> Shouldn't we return -EWOULDBLOCK by default?
> QEMU does not use that path with GICv2m but in kvm_set_routing_entry() 
> I
> don't see any check related to the ITS.

Fair enough. I really don't anticipate anyone to be using
KVM_IRQ_ROUTING_MSI with anything but the ITS, but who knows,
people are crazy! ;-)

>> +
>>  		kvm_populate_msi(e, &msi);
>> -		if (!vgic_its_inject_cached_translation(kvm, &msi))
>> -			return 0;
>> +		return vgic_its_inject_cached_translation(kvm, &msi);
> 
>>  	}
>> 
>> -	return -EWOULDBLOCK;
>> +	case KVM_IRQ_ROUTING_IRQCHIP:
>> +		/* Injecting SPIs is always possible in atomic context */
>> +		return vgic_irqfd_set_irq(e, kvm, irq_source_id, 1, line_status);
> what about the 	mutex_lock(&kvm->lock) called from within
> vgic_irqfd_set_irq/kvm_vgic_inject_irq/vgic_lazy_init

Holy crap. The lazy GIC init strikes again :-(.
How about this on top of the existing patch:

diff --git a/arch/arm64/kvm/vgic/vgic-irqfd.c 
b/arch/arm64/kvm/vgic/vgic-irqfd.c
index 11a9f81115ab..6e5ca04d5589 100644
--- a/arch/arm64/kvm/vgic/vgic-irqfd.c
+++ b/arch/arm64/kvm/vgic/vgic-irqfd.c
@@ -115,19 +115,23 @@ int kvm_arch_set_irq_inatomic(struct 
kvm_kernel_irq_routing_entry *e,
  		struct kvm_msi msi;

  		if (!vgic_has_its(kvm))
-			return -EINVAL;
+			break;

  		kvm_populate_msi(e, &msi);
  		return vgic_its_inject_cached_translation(kvm, &msi);
  	}

  	case KVM_IRQ_ROUTING_IRQCHIP:
-		/* Injecting SPIs is always possible in atomic context */
+		/*
+		 * Injecting SPIs is always possible in atomic context
+		 * as long as the damn vgic is initialized.
+		 */
+		if (unlikely(!vgic_initialized(kvm)))
+			break;
  		return vgic_irqfd_set_irq(e, kvm, irq_source_id, 1, line_status);
-
-	default:
-		return -EWOULDBLOCK;
  	}
+
+	return -EWOULDBLOCK;
  }

  int kvm_vgic_setup_default_irq_routing(struct kvm *kvm)

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

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