Re: [PATCH v2] KVM: s390: Improve floating IRQ injection behavior

[Halil Pasic <pasic@linux.ibm.com>]

On Mon, 17 Aug 2026 15:22:41 +0200
Christian Borntraeger <borntraeger@linux.ibm.com> wrote:

> >> +	irq_pend_mask = inti_to_irq_pend_mask(inti);
> >>   	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;  
> > 
> > [Severity: High]
> > Does this code drop the interrupt kick entirely if all vCPUs currently
> > have their interrupt masks closed?  
> 
> I think this is a corner case but still a valid finding. We can probably consider this
> slowpath and wakeup/set cpuflags for ALL cpus? maybe after doing 2 rounds instead of one?

With GISA, I think the FW is supposed to deliver the floating interrupts
without dropping the vCPU out of SIE. I'm not 100% sure but I think we
can rely on that mechanism for the CPUs that are in SIE. Without GISA,
I think, it is reasonable to assume that vCPUs don't keep running for
ever. It has been a while since I have looked at this code, but I think
the SIE exit path would catch this. If that is true we would not really
lose initiative, but just see delayed interrupt delivery.

Sleeping vCPUs on the other hand are not of interest in this context, I
think.

Opinions?

Regards,
Halil