Re: [PATCH v4 1/3] KVM: s390: Improve floating IRQ injection behavior

[Christian Borntraeger <borntraeger@linux.ibm.com>]

Am 20.08.26 um 14:12 schrieb Janosch Frank:
> Floating IRQs can be handled by any VCPU that opened its masks. The
> current design does not check if the mask is open when a floating IRQ
> is injected via the FLIC. It will wakeup the last VCPU that went
> sleeping hoping it's the correct one.
> 
> Improve this by checking if the VCPU has pending IRQs and if not try
                                        can deliver that particular IRQ> to find another VCPU which can take the IRQ.
> 
> This is not the final fix around this topic but we'll eventually
> inject a pending IRQ. The current code can easily deadlock a VM and
> with this fix we work around that.


One question below.


probably not the last version of that code ara but certainly a necessary
fix in the right direction.

Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>

We should apply this and add it to kvms390. We will get some
performance regression testing via the next kernel then and wait
for the results before sending out a pull request.


> [...]
>   /*
>    * Find a destination VCPU for a floating irq and kick it.
>    */
> -static void __floating_irq_kick(struct kvm *kvm, u64 type)
> +static void __floating_irq_kick(struct kvm *kvm, u64 type, int isc)
>   {
>   	struct kvm_vcpu *dst_vcpu;
>   	int sigcpu, online_vcpus, nr_tries = 0;
> +	u64 irq_pend_mask;
> +	unsigned long i;
>   
>   	online_vcpus = atomic_read(&kvm->online_vcpus);
>   	if (!online_vcpus)
>   		return;
>   
> +	irq_pend_mask = inti_to_irq_pend_mask(type, isc);
>   	for (sigcpu = kvm->arch.float_int.last_sleep_cpu; ; sigcpu++) {
>   		sigcpu %= online_vcpus;
>   		dst_vcpu = kvm_get_vcpu(kvm, sigcpu);
> -		if (!is_vcpu_stopped(dst_vcpu))
> +		if (!is_vcpu_stopped(dst_vcpu) &&
> +		    deliverable_irqs(dst_vcpu) & irq_pend_mask)
>   			break;
>   		/* avoid endless loops if all vcpus are stopped */
> -		if (nr_tries++ >= online_vcpus)
> -			return;
> +		if (nr_tries++ >= online_vcpus * 2) {

why 2 rounds?

> +			dst_vcpu = NULL;
> +			break;
> +		}
>   	}
>