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

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

On Tue, 18 Aug 2026 17:14:52 +0200
Christian Borntraeger <borntraeger@linux.ibm.com> wrote:

> Am 18.08.26 um 16:32 schrieb Halil Pasic:
> > 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.  
> This is all corner case handling. Imagine one CPU running with IO disabled
> and all other CPUs sleeping. If now the "opportunistic" wakeup fails the
> GISA IO interrupt will never be delivered unless there is another wakeup.
> In reality this is a will not happen, but see the latest kvm unit test
> patch from Janosch and it might also create latencies, the "pick one CPU
> to deliver and wake it up if normal processing does not work" obviously
> has a hole in specific cases.

Right, but that is not the "if all vCPUs currently have their interrupt
masks closed" case that Sashiko is talking about. Or did I misunderstand
that?

Yes, I agree there are holes, and I was hoping to contribute to a better
understanding on where the holes actually are and what are the
implications of those holes.

Regards,
Halil